reconintra.c fix warning error

Fixes warning error introduced by : aom/5942

Change-Id: I1e2c013a3f8859bbcf4548a67347011f4b0fb650
diff --git a/av1/common/reconintra.c b/av1/common/reconintra.c
index 809e9b4..049f855 100644
--- a/av1/common/reconintra.c
+++ b/av1/common/reconintra.c
@@ -943,7 +943,7 @@
 #if CONFIG_TX64X64
     case 64: return TX_64X64;
 #endif  // CONFIG_TX64X64
-    default: assert(0);
+    default: assert(0); return NULL;
   }
 }