Bug#13437900 - VALGRIND REPORTS A LEAK FOR REPL_IGNORE_SERVER_IDS

There was memory leak when running some tests on PB2.
The reason of the failure is an early return from change_master()
that was supposed to deallocate a dyn-array.

Actually the same bug58915 was fixed in trunk with relocating the dyn-array
destruction into THD::cleanup_after_query() which can't be bypassed.
The current patch backports magne.mahre@oracle.com-20110203101306-q8auashb3d7icxho
and adds two optimizations: were done: the static buffer for the dyn-array to base on,
and the array initialization is called precisely when it's necessary rather than
per each CHANGE-MASTER as before.

sql/sql_lex.cc

@@ -2351,6 +2351,7 @@ LEX::LEX()
                          plugins_static_buffer,
                          INITIAL_LEX_PLUGIN_LIST_SIZE, 
                          INITIAL_LEX_PLUGIN_LIST_SIZE);
+  memset(&mi, 0, sizeof(LEX_MASTER_INFO));
   reset_query_tables_list(TRUE);
 }