luci/buildbucket/build.go: Fix panic
luci/buildbucket/build.go: Fix panic

Bug: 782926
Change-Id: I796dbf6436adb8bb164e3c63500d78c731bd57f7
Reviewed-on: https://chromium-review.googlesource.com/759456
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go