* keyboard.c (make_lispy_event): Fix problem in integer overflow.

* keyboard.c (make_lispy_event): Fix problem in integer overflow.

Don't assume that the difference between two unsigned long values
can fit into an integer.  At this point, we know button_down_time
<= event->timestamp, so the difference must be nonnegative, so
there's no need to cast the result if double-click-time is
nonnegative, as it should be; check that it's nonnegative, just in
case.  This bug is triggered when events are more than 2**31 ms
apart (about 25 days).