MDEV-19740 Debug build of 10.3.15 FTBFS

Fix debug build failing with error:

extended initializer lists only available with -std=c++11 or -std=gnu++11

sql/sql_explain.cc

@@ -444,7 +444,8 @@ uint Explain_union::make_union_table_name(char *buf)
       break;
     default:
       DBUG_ASSERT(0);
-      type= {NULL, 0};
+      type.str= NULL;
+      type.length= 0;
   }
   memcpy(buf, type.str, (len= (uint)type.length));