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)

diff --git a/src/enc/backward_references_enc.c b/src/enc/backward_references_enc.c
index 3923018..b29fc9e 100644
--- a/src/enc/backward_references_enc.c
+++ b/src/enc/backward_references_enc.c

@@ -910,13 +910,14 @@
       quality >= 25) {
     const VP8LHashChain* const hash_chain_tmp =
         (lz77_type_best == kLZ77Standard) ? hash_chain : &hash_chain_box;
-    if (VP8LBackwardReferencesTraceBackwards(width, height, argb, *cache_bits,
-                                             hash_chain_tmp, best, worst)) {
-      double bit_cost_trace;
-      VP8LHistogramCreate(histo, worst, *cache_bits);
-      bit_cost_trace = VP8LHistogramEstimateBits(histo);
-      if (bit_cost_trace < bit_cost_best) best = worst;
+    double bit_cost_trace;
+    if (!VP8LBackwardReferencesTraceBackwards(width, height, argb, *cache_bits,
+                                              hash_chain_tmp, best, worst)) {
+      goto Error;
     }
+    VP8LHistogramCreate(histo, worst, *cache_bits);
+    bit_cost_trace = VP8LHistogramEstimateBits(histo);
+    if (bit_cost_trace < bit_cost_best) best = worst;
   }
 
   BackwardReferences2DLocality(width, best);