[autoroll] Fix out-of-bounds in github repo manager
[autoroll] Fix out-of-bounds in github repo manager

Change-Id: I2ccfded39305e2d25ca3a27a9c74c07a4dda9133
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/256022
Reviewed-by: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot