MDEV-20926 UBSAN: load of value 165, which is not a valid value for type bool

Basicaly it's an uninitialized read. 165 is 0xa5 which comes from TRASH_ALLOC()

Fix by calling a class ctor which initializes problematic
TMP_TABLE_PARAM::force_copy_fields field

sql/sql_show.cc

@@ -7708,8 +7708,7 @@ TABLE *create_schema_table(THD *thd, TABLE_LIST *table_list)
     item->maybe_null= (fields_info->field_flags & MY_I_S_MAYBE_NULL);
     field_count++;
   }
-  TMP_TABLE_PARAM *tmp_table_param =
-    (TMP_TABLE_PARAM*) (thd->alloc(sizeof(TMP_TABLE_PARAM)));
+  TMP_TABLE_PARAM *tmp_table_param = new TMP_TABLE_PARAM;
   tmp_table_param->init();
   tmp_table_param->table_charset= cs;
   tmp_table_param->field_count= field_count;