concurrently execute the statement XA START 'x', then mysqld
server could crash.

sql/sql_class.cc:
  xid_cache_insert: added checking for element in cache before
  insert it, return TRUE if such element already exists.
sql/sql_parse.cc:
  mysql_execute_command modified:
  * sequence of calls to xid_cache_search(..)/xid_cache_insert(...)
  replaced by call to xid_cache_insert(...) in alternative
  'case SQLCOM_XA_START:'
  * added comment to alternative 'case SQLCOM_XA_COMMIT:'.