The constructor of Rotate_log_event used when we are rotating our binlog or
relay log, should not assume that there is a nonzero THD available.
For example, when we are reacting to SIGHUP, the THD is 0.
In fact we don't need to use the THD in this constructor;
we can do like for Stop_log_event, and use the minimal Log_event
constructor.
If we were allowed to put Unix-specific commands in the testsuite,
I'd add a test for this (<sigh>).