replace NULL with TX_INVALID

This commit fixes a build error where returned value does not match
defined return type.

Change-Id: I1d2c879d0cf7d90fcc7105683695a371c9fe9f58

av1/common/reconintra.c

@@ -943,7 +943,7 @@ static INLINE TX_SIZE get_txsize_from_blocklen(int bs) {
 #if CONFIG_TX64X64
     case 64: return TX_64X64;
 #endif  // CONFIG_TX64X64
-    default: assert(0); return NULL;
+    default: assert(0); return TX_INVALID;
   }
 }