Another fix to timer handling.

Robert de Bath points out that failure to remove the timer whose
callback returned FALSE may not have been the cause of runaway timer
explosion; another possibility is that a function called from
timer_trigger()'s call to run_timers() has already set a timer up by
the time run_timers() returns, and then we set another one up on top
of it. Fix that too.


git-svn-id: svn://svn.tartarus.org/sgt/putty@10206 cda61777-01e9-0310-a592-d414129be87e