Remove mem_alloc_error()

As thd->alloc() and new automatically calls my_error(ER_OUTOFMEORY)
there is no reason to call mem_alloc_error()

Other things:
- Fixed bug in mysql_unpack_partition() where lex.part_info was
  changed even if it would be a null pointer

sql/sql_partition_admin.cc

@@ -342,10 +342,7 @@ static bool exchange_name_with_ddl_log(THD *thd,
   DBUG_ENTER("exchange_name_with_ddl_log");
 
   if (unlikely(!(file= get_new_handler(NULL, thd->mem_root, ht))))
-  {
-    mem_alloc_error(sizeof(handler));
     DBUG_RETURN(TRUE);
-  }
 
   /* prepare the action entry */
   exchange_entry.entry_type=   DDL_LOG_ENTRY_CODE;