If you call mysql_stmt_send_long_data while the MYSQL* is in the wrong state, it returns the fact that there is an error, but no error code.
See line 1826 of my_stmt.c.

To correct, I think the conditional on line 1816 needs to be changed from:
if (stmt->mysql->status== MYSQL_STATUS_READY && (length || !stmt->params[param_number].long_data_used))
{

To:
if (stmt->mysql->status != MYSQL_STATUS_READY)
SET_CLIENT_STMT_ERROR(stmt, CR_COMMANDS_OUT_OF_SYNC, SQLSTATE_UNKNOWN, 0);
DBUG_RETURN(1);
}

if (length || !stmt->params[param_number].long_data_used))
{