Fix a crash I introduced by the last push.

sql/sql_prepare.cc

@@ -2447,6 +2447,7 @@ Prepared_statement::~Prepared_statement()
   if (cursor)
     cursor->Cursor::~Cursor();
   free_items();
+  if (cursor)
     free_root(cursor->mem_root, MYF(0));
   delete lex->result;
 }