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>).


sql/log.cc:
  A comment to warn that thd can be 0.
  The part about LOG_EVENT_FORCED_ROTATE_F is just to avoid segfault;
  this flag is already removed in 4.1 anyway.
sql/log_event.cc:
  A comment.
sql/log_event.h:
  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.
  This fixes BUG#2045 
  "Sending SIGHUP to mysqld crashes it if running with --log-bin"