Problem:
========
A master can segfault if it can't set up decryption for its binary
log during a binlog dump with Using_Gtid=Slave_Pos. If slave
connects using GTID mode, the master will call into
log.cc::get_gtid_list_event(), which iterate through binlog events
looking for a Gtid_list_log_event. On an encrypted binlog that the
master cannot decrypt, the first event will be a
START_ENCRYPTION_EVENT which will call into the following decryption branch

if (fdle->start_decryption((Start_encryption_log_event*) ev))
  errormsg= ‘Could not set up decryption for binlog.’;

The event iteration however, does not stop in spite of this error.
The master will try to read the next event, but segfault while
trying to decrypt it because decryption failed to initialize.

Solution:
========
Break the event iteration if decryption cannot be set up.

Reviewed By:
============
Andrei Elkin <andrei.elkin@mariadb.com>