Silently ignore timer events from timers which were just stopped.

Silently ignore timer events from timers which were just stopped.

An assert in wxTimerWndProc() could be provoked by valid user code which
simply started and stopped the timers quickly enough because a WM_TIMER could
have been already generated just before we stopped the timer.

Simply ignore events from unknown timer under assumption that they must come
from the recently stopped ones instead of asserting.

Ideally we'd somehow distinguish between the situation described above and the
really bogus events which could indicate bugs in wx code or a change in
behaviour in a future version of Windows but there is no easy way to do it so
for now just settle for not asserting in normal case.

Closes #10052.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66228 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775