fuzz: Panic during local variable rewriting stage

[tsandall]

Test file:

package d c({({0|t:=0}):0})

Panic:

panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x89cbda]

goroutine 1 [running]:
github.com/open-policy-agent/opa/ast.(*Compiler).compile.func1()
        /home/torin/src/opa/ast/compile.go:881 +0x67
panic(0xbfc020, 0x11f7350)
        /usr/local/go/src/runtime/panic.go:969 +0x175
github.com/open-policy-agent/opa/ast.(*Compiler).rewriteLocalVars.func1.1(0x0, 0x250eef6a57e40609)
        /home/torin/src/opa/ast/compile.go:1120 +0x15a
github.com/open-policy-agent/opa/ast.WalkTerms.func1(0xc8ce80, 0x0, 0xc000320ba0)
        /home/torin/src/opa/ast/visit.go:181 +0x42
github.com/open-policy-agent/opa/ast.(*GenericVisitor).Walk(0xc0003060c0, 0xc8ce80, 0x0)
        /home/torin/src/opa/ast/visit.go:270 +0x58
github.com/open-policy-agent/opa/ast.(*GenericVisitor).Walk.func1(0xc00031e960, 0xc00031eaa0)
        /home/torin/src/opa/ast/visit.go:340 +0x97
github.com/open-policy-agent/opa/ast.(*object).Foreach.func1(0xc00031e960, 0xc00031eaa0, 0x40e218, 0x30)
        /home/torin/src/opa/ast/term.go:1810 +0x39
github.com/open-policy-agent/opa/ast.(*object).Iter(0xc000320bd0, 0xc000305ca8, 0xc000305d90, 0x89ccd5)
        /home/torin/src/opa/ast/term.go:1788 +0x7a
github.com/open-policy-agent/opa/ast.(*object).Foreach(0xc000320bd0, 0xc000305d98)
        /home/torin/src/opa/ast/term.go:1809 +0x53
github.com/open-policy-agent/opa/ast.(*GenericVisitor).Walk(0xc0003060c0, 0xcb3cc0, 0xc000320bd0)
        /home/torin/src/opa/ast/visit.go:338 +0x994
github.com/open-policy-agent/opa/ast.(*GenericVisitor).Walk(0xc0003060c0, 0xc8ce80, 0xc00031e940)
        /home/torin/src/opa/ast/visit.go:332 +0xbc7
github.com/open-policy-agent/opa/ast.(*GenericVisitor).Walk(0xc0003060c0, 0xc3f580, 0xc00031eee0)
        /home/torin/src/opa/ast/visit.go:312 +0xc50
github.com/open-policy-agent/opa/ast.(*GenericVisitor).Walk(0xc0003060c0, 0xc73260, 0xc0002165a0)
        /home/torin/src/opa/ast/visit.go:299 +0x8a5
github.com/open-policy-agent/opa/ast.WalkTerms(0xc73260, 0xc0002165a0, 0xc000306210)
        /home/torin/src/opa/ast/visit.go:185 +0x70
github.com/open-policy-agent/opa/ast.(*Compiler).rewriteLocalVars.func1(0xc000215480, 0x1249301)
        /home/torin/src/opa/ast/compile.go:1117 +0xce
github.com/open-policy-agent/opa/ast.WalkRules.func1(0xc73340, 0xc000215480, 0xc00031eec0)
        /home/torin/src/opa/ast/visit.go:229 +0x56
github.com/open-policy-agent/opa/ast.(*GenericVisitor).Walk(0xc0003064d0, 0xc73340, 0xc000215480)
        /home/torin/src/opa/ast/visit.go:270 +0x58
github.com/open-policy-agent/opa/ast.(*GenericVisitor).Walk(0xc0003064d0, 0xc4a0a0, 0xc000216550)
        /home/torin/src/opa/ast/visit.go:281 +0x59c
github.com/open-policy-agent/opa/ast.WalkRules(0xc4a0a0, 0xc000216550, 0xc000306538)
        /home/torin/src/opa/ast/visit.go:238 +0x70
github.com/open-policy-agent/opa/ast.(*Compiler).rewriteLocalVars(0xc0003120f0)
        /home/torin/src/opa/ast/compile.go:1103 +0xe5
github.com/open-policy-agent/opa/ast.(*Compiler).runStage(0xc0003120f0, 0xcf48e2, 0x20, 0xc0002091f0)
        /home/torin/src/opa/ast/compile.go:866 +0x4f
github.com/open-policy-agent/opa/ast.(*Compiler).compile(0xc0003120f0)
        /home/torin/src/opa/ast/compile.go:886 +0x11d
github.com/open-policy-agent/opa/ast.(*Compiler).Compile(0xc0003120f0, 0xc000306888)
        /home/torin/src/opa/ast/compile.go:351 +0x2c5
github.com/open-policy-agent/opa/bundle.writeModules(0xdd6340, 0xc000138010, 0xddcfa0, 0xc000214d00, 0xdc4a60, 0xc000308450, 0xc0003120f0, 0xddb540, 0xc00027caa0, 0x0, ...)
        /home/torin/src/opa/bundle/store.go:397 +0x5ed
github.com/open-policy-agent/opa/bundle.activateBundles(0xc000307068, 0xc00027caa0, 0xddcfa0)
        /home/torin/src/opa/bundle/store.go:243 +0x866
github.com/open-policy-agent/opa/bundle.Activate(...)
        /home/torin/src/opa/bundle/store.go:161
github.com/open-policy-agent/opa/internal/runtime/init.InsertAndCompile(0xdd6340, 0xc000138010, 0xddcfa0, 0xc000214d00, 0xdc4a60, 0xc000308450, 0xc000205b00, 0xc000205b30, 0x0, 0x0, ...)
        /home/torin/src/opa/internal/runtime/init/init.go:67 +0x313
github.com/open-policy-agent/opa/plugins.(*Manager).Init.func1(0xdc4a60, 0xc000308450, 0xc000138010, 0xc000209190)
        /home/torin/src/opa/plugins/plugins.go:255 +0x158
github.com/open-policy-agent/opa/storage.Txn(0xdd6340, 0xc000138010, 0xddcfa0, 0xc000214d00, 0x1, 0xc00020a340, 0xc000307328, 0x0, 0x0)
        /home/torin/src/opa/storage/storage.go:95 +0xe9
github.com/open-policy-agent/opa/plugins.(*Manager).Init(0xc000312000, 0xdd6340, 0xc000138010, 0xc00024e030, 0x24)
        /home/torin/src/opa/plugins/plugins.go:253 +0x105
github.com/open-policy-agent/opa/runtime.NewRuntime(0xdd6340, 0xc000138010, 0xc00024e030, 0x24, 0xc000206560, 0xc000206580, 0x0, 0x0, 0x0, 0x0, ...)
        /home/torin/src/opa/runtime/runtime.go:251 +0x408
github.com/open-policy-agent/opa/cmd.initRuntime(0xdd6340, 0xc000138010, 0x0, 0x0, 0xc000206560, 0xc000206580, 0x0, 0x0, 0x0, 0x0, ...)
        /home/torin/src/opa/cmd/run.go:258 +0x545
github.com/open-policy-agent/opa/cmd.init.8.func1(0xc00021d180, 0xc0002084e0, 0x1, 0x1)
        /home/torin/src/opa/cmd/run.go:156 +0xea
github.com/spf13/cobra.(*Command).execute(0xc00021d180, 0xc0002084b0, 0x1, 0x1, 0xc00021d180, 0xc0002084b0)
        /home/torin/src/opa/vendor/github.com/spf13/cobra/command.go:766 +0x2c2
github.com/spf13/cobra.(*Command).ExecuteC(0x1203480, 0xc000068778, 0xc000113f78, 0x4062a5)
        /home/torin/src/opa/vendor/github.com/spf13/cobra/command.go:852 +0x2fe
github.com/spf13/cobra.(*Command).Execute(...)
        /home/torin/src/opa/vendor/github.com/spf13/cobra/command.go:800
main.main()
        /home/torin/src/opa/main.go:15 +0x31