Panic if 'dep ensure' is interrupted

[Civil]

dep ensure
^CSignal received: waiting for 1 ops to complete...
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0x719844]

goroutine 397 [running]:
github.com/golang/dep/vendor/github.com/sdboyer/gps.(*monitoredCmd).run(0xc4201b3920, 0xa69900, 0xc4203af5e0, 0x0, 0x0)
        /root/go/src/github.com/golang/dep/vendor/github.com/sdboyer/gps/cmd.go:71 +0x344
github.com/golang/dep/vendor/github.com/sdboyer/gps.(*monitoredCmd).combinedOutput(0xc4201b3920, 0xa69900, 0xc4203af5e0, 0xc42044c000, 0x10, 0x1c, 0xc42044c000, 0x10)
        /root/go/src/github.com/golang/dep/vendor/github.com/sdboyer/gps/cmd.go:85 +0x43
github.com/golang/dep/vendor/github.com/sdboyer/gps.(*gitSource).listVersions(0xc4201905d0, 0xa69900, 0xc4203af5e0, 0x410ba8, 0x10, 0x826e20, 0xc4203fd901, 0xc4201905e0)
        /root/go/src/github.com/golang/dep/vendor/github.com/sdboyer/gps/vcs_source.go:166 +0x358
github.com/golang/dep/vendor/github.com/sdboyer/gps.maybeGitSource.try.func1(0xa69900, 0xc4203af5e0, 0xa69680, 0xc42001d040)
        /root/go/src/github.com/golang/dep/vendor/github.com/sdboyer/gps/maybe_source.go:97 +0x71
github.com/golang/dep/vendor/github.com/sdboyer/gps.(*supervisor).do(0xc420259c00, 0xa696c0, 0xc420078640, 0x88a7f3, 0xc, 0x1, 0xc4203fda68, 0x18, 0x819960)
        /root/go/src/github.com/golang/dep/vendor/github.com/sdboyer/gps/source_manager.go:485 +0xd5
github.com/golang/dep/vendor/github.com/sdboyer/gps.maybeGitSource.try(0xc4203f5b00, 0xa696c0, 0xc420078640, 0xc420257890, 0x10, 0xa6c600, 0xc4203ec200, 0xc420259c00, 0x6, 0xc4202bcc01, ...)
        /root/go/src/github.com/golang/dep/vendor/github.com/sdboyer/gps/maybe_source.go:101 +0x293
github.com/golang/dep/vendor/github.com/sdboyer/gps.maybeSources.try(0xc4203ec1c0, 0x4, 0x4, 0xa696c0, 0xc420078640, 0xc420257890, 0x10, 0xa6c600, 0xc4203ec200, 0xc420259c00, ...)
        /root/go/src/github.com/golang/dep/vendor/github.com/sdboyer/gps/maybe_source.go:31 +0xf1
github.com/golang/dep/vendor/github.com/sdboyer/gps.(*maybeSources).try(0xc42000c5e0, 0xa696c0, 0xc420078640, 0xc420257890, 0x10, 0xa6c600, 0xc4203ec200, 0xc420259c00, 0xc420249658, 0xc4202bcd30, ...)
        <autogenerated>:183 +0xc7
github.com/golang/dep/vendor/github.com/sdboyer/gps.(*sourceGateway).require(0xc4203e9500, 0xa696c0, 0xc420078640, 0x1, 0x40, 0x0, 0x0)
        /root/go/src/github.com/golang/dep/vendor/github.com/sdboyer/gps/source.go:436 +0x243
github.com/golang/dep/vendor/github.com/sdboyer/gps.(*sourceGateway).sourceURL(0xc4203e9500, 0xa696c0, 0xc420078640, 0x0, 0x0, 0x0, 0x0)
        /root/go/src/github.com/golang/dep/vendor/github.com/sdboyer/gps/source.go:404 +0xbc
github.com/golang/dep/vendor/github.com/sdboyer/gps.(*sourceCoordinator).setUpSourceGateway(0xc420016180, 0xa696c0, 0xc420078640, 0xc42019bf80, 0x21, 0xc4203e9440, 0xc4203e94a0)
        /root/go/src/github.com/golang/dep/vendor/github.com/sdboyer/gps/source.go:161 +0x8f8
created by github.com/golang/dep/vendor/github.com/sdboyer/gps.(*sourceCoordinator).getSourceGatewayFor
        /root/go/src/github.com/golang/dep/vendor/github.com/sdboyer/gps/source.go:88 +0x3a9
make: *** [dep] Error 2

I'm on commit bca5bdb, so that should include the fixes you've made about e.x. #391.

[sdboyer]

Oh, derp! I was just noticing we didn't have a test case for this last night, but didn't get round to writing one. And, now I see the bug. Will fix it in a jiffy.