GetBackwardReferences: fail on alloc error

previously failures in the call to
VP8LBackwardReferencesTraceBackwards() would be ignored which, though it
wouldn't result in a crash, would produce non-deterministic output

Change-Id: Id9890a60883c3270ec75e968506d46eea32b76d4
(cherry picked from commit e3cfafaf719c2e163d3548d7a415da96fdff714f)
(cherry picked from commit 20ef03ee351d4ff03fc5ff3ec4804a879d1b9d5c)
(cherry picked from commit 89e226a3c70ae04fe8e1884aa0b291f39adb2fdf)