backupccl: NPE during RESTORE rollback

[pbardea]

While cancelling a RESTORE job, the coordinator crashes with the following NPE:

I200805 02:47:06.560063 1551 jobs/registry.go:1240  job 578435388608741377: resuming execution
I200805 02:47:06.564606 183009 jobs/registry.go:842  [n1] job 578435388608741377: stepping through state reverting with error: job canceled by user
E200805 02:47:06.568147 183009 util/log/crash_reporting.go:138  [n1] a panic has occurred!
runtime error: invalid memory address or nil pointer dereference
(1) attached stack trace
  | runtime.gopanic
  |     /usr/local/go/src/runtime/panic.go:969
  | runtime.panicmem
  |     /usr/local/go/src/runtime/panic.go:212
  | runtime.sigpanic
  |     /usr/local/go/src/runtime/signal_unix.go:695
  | github.com/cockroachdb/cockroach/pkg/ccl/backupccl.(*restoreResumer).dropTables
  |     /go/src/github.com/cockroachdb/cockroach/pkg/ccl/backupccl/restore_job.go:1188
  | github.com/cockroachdb/cockroach/pkg/ccl/backupccl.(*restoreResumer).OnFailOrCancel.func1
  |     /go/src/github.com/cockroachdb/cockroach/pkg/ccl/backupccl/restore_job.go:1172
  | github.com/cockroachdb/cockroach/pkg/kv.(*DB).Txn.func1
  |     /go/src/github.com/cockroachdb/cockroach/pkg/kv/db.go:707
  | github.com/cockroachdb/cockroach/pkg/kv.(*Txn).exec
  |     /go/src/github.com/cockroachdb/cockroach/pkg/kv/txn.go:811
  | github.com/cockroachdb/cockroach/pkg/kv.(*DB).Txn
  |     /go/src/github.com/cockroachdb/cockroach/pkg/kv/db.go:706
  | github.com/cockroachdb/cockroach/pkg/ccl/backupccl.(*restoreResumer).OnFailOrCancel
  |     /go/src/github.com/cockroachdb/cockroach/pkg/ccl/backupccl/restore_job.go:1164
  | github.com/cockroachdb/cockroach/pkg/jobs.(*Registry).stepThroughStateMachine
  |     /go/src/github.com/cockroachdb/cockroach/pkg/jobs/registry.go:911
  | github.com/cockroachdb/cockroach/pkg/jobs.(*Registry).resume.func1
  |     /go/src/github.com/cockroachdb/cockroach/pkg/jobs/registry.go:986
  | github.com/cockroachdb/cockroach/pkg/util/stop.(*Stopper).RunAsyncTask.func1
  |     /go/src/github.com/cockroachdb/cockroach/pkg/util/stop/stopper.go:347
  | runtime.goexit
  |     /usr/local/go/src/runtime/asm_amd64.s:1373
Wraps: (2) runtime error: invalid memory address or nil pointer dereference
Error types: (1) *withstack.withStack (2) runtime.errorString

On this line: https://github.com/cockroachdb/cockroach/blob/master/pkg/ccl/backupccl/restore_job.go#L1188