vp9: sync workers at the start of decode_tiles_mt()

prevents any problems resuming decode after decoding a corrupt frame

Change-Id: Ib7eb1b5c062aebe71074fef1ece32a32822c16be

vp9/decoder/vp9_decodeframe.c

@@ -994,6 +994,7 @@ static const uint8_t *decode_tiles_mt(VP9Decoder *pbi,
 
   // Reset tile decoding hook
   for (n = 0; n < num_workers; ++n) {
+    winterface->sync(&pbi->tile_workers[n]);
     pbi->tile_workers[n].hook = (VP9WorkerHook)tile_worker_hook;
   }