Add assert(tx_type == DCT_DCT) when FIXED_TX_TYPE.

Change-Id: I9bc925ab9b93f5318eb3f1aa3401ff3de2851fef
diff --git a/av1/decoder/decodemv.c b/av1/decoder/decodemv.c
index fd8d10c..22d34f5 100644
--- a/av1/decoder/decodemv.c
+++ b/av1/decoder/decodemv.c
@@ -1043,6 +1043,9 @@
     }
 #endif  // CONFIG_EXT_TX
   }
+#if FIXED_TX_TYPE
+  assert(mbmi->tx_type == DCT_DCT);
+#endif
 }
 
 #if CONFIG_INTRABC