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 <[email protected]>

Author: kevinburke

maintner/github.go

@@ -1951,6 +1951,7 @@ func (c *Corpus) parseGithubRefs(gerritProj string, commitMsg string) []GitHubIs
 	2017/03/30 21:42:31 matches: [["GoogleCloudPlatform/gcloud-golang#262" "GoogleCloudPlatform" "gcloud-golang" "262"]]
 	2017/03/30 21:42:31 matches: [["GoogleCloudPlatform/google-cloud-go#481" "GoogleCloudPlatform" "google-cloud-go" "481"]]
 	*/
+	c.initGithub()
 	github := c.GitHub()
 	refs := make([]GitHubIssueRef, 0, len(ms))
 	for _, m := range ms {