[gold] Fix panic in diffcalculation
[gold] Fix panic in diffcalculation

Bug: skia:10582
Change-Id: I258c7f748131d7cadc91c3502d13017e007f6be1
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/433457
Reviewed-by: Leandro Lovisolo <f558143c2568be8d55575d13f30a1fc5cb8009d5@google.com>
Commit-Queue: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@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