Manual merge from mysql-5.1 for Bug#11764168 (56976: Severe denial

of service in prepared statements).

sql/sql_prepare.cc:
  At mysql_stmt_get_longdata(): instead of pushing an internal
  error handler (as done in 5.1-tree) we save, set and restore
  the statement's diagnostics area and warning info.

mysql-test/t/variables.test

@@ -1281,6 +1281,11 @@ SET @@global.max_join_size=0;
 SET @@global.key_buffer_size=0;
 SET @@global.key_cache_block_size=0;
 
+#
+# Bug#56976: added new start-up parameter
+#
+select @@max_long_data_size;
+
 # cleanup
 SET @@global.max_binlog_cache_size=DEFAULT;
 SET @@global.max_join_size=DEFAULT;