maintner: fix another panic

You can trigger this by creating a Corpus and only adding Gerrit
repositories to it. These call parseGithubRefs, but c.github.repos
is never initialized, so that function panics when you try to get
c.github.repos.

I'm a little worried I've triggered two of these recently; maybe it's
worth it to initialize the data structures earlier instead of
lazy-loading them.

Change-Id: I8f80c0947d320aef0dd9391586c306460ac77eea
Reviewed-on: https://go-review.googlesource.com/41305
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed