In multi_update::send_data(), the counter of matched rows was not correctly incremented, when during insertion of a new row to a temporay table it had to be converted from HEAP to MyISAM.

This fix changes the logic to increment the counter of matched rows in the following cases:
1. If the error returned from write_row() is zero.
2. If the error returned from write_row() is non-zero, is neither HA_ERR_FOUND_DUPP_KEY nor HA_ERR_FOUND_DUPP_UNIQUE, and a call to create_myisam_from_heap() succeeds.