cmd/gerritbot: always die when initCorpus fails
cmd/gerritbot: always die when initCorpus fails

If an error were to occur in corpusUpdateLoop's call of
initCorpus, b.corpus would have been nil after the continue,
causing a nil pointer dereference.

Change-Id: I0257fca95141eb9e9c1a48466f5d95df87ad5e76
Reviewed-on: https://go-review.googlesource.com/96200
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build