MDEV-26504 THD::copy_db_to() fails to return true if THD::db is null

THD::copy_db_to(): Always return true if the output parameter
was left uninitialized. This fixes a regression that was caused
by commit 7d0d934c (MDEV-16473).

MariaDB Server 10.3 and later were unaffected by this bug
thanks to commit a7e352b5.

Possibly this bug only affects mysql_list_fields()
in the Embedded Server (libmysqld).

This bug was found by GCC 11.2.0 in CMAKE_BUILD_TYPE=RelWithDebInfo.

sql/sql_class.h

@@ -4003,11 +4003,9 @@ class THD :public Statement,
         for any CTE references.
       */
       if (!lex->with_cte_resolution)
-      {
         my_message(ER_NO_DB_ERROR, ER(ER_NO_DB_ERROR), MYF(0));
       return TRUE;
     }
-    }
     else
     {
      *p_db= strmake(db, db_length);