Fix build failure with GCC 10.2

GCC is smart enough to detect that the address of a local variable is
returned. Since this appears to be code used for a debug assertion, we can
just return a null pointer.

query_classifier/qc_sqlite/sqlite-src-3110100/src/select.c

@@ -122,7 +122,9 @@ Select *sqlite3SelectNew(
   pNew = sqlite3DbMallocRawNN(db, sizeof(*pNew) );
   if( pNew==0 ){
     assert( db->mallocFailed );
+#ifndef MAXSCALE
     pNew = &standin;
+#endif
   }
   if( pEList==0 ){
 #ifndef MAXSCALE