Avoid race condition when handling a SIGWINCH signal

Avoid race condition when handling a SIGWINCH signal

The control variable win_resized must be cleared before responding to
it.

Otherwise there is a small window where another SIGWINCH might occur in
between the handling of an earlier SIGWINCH and the clearing of
win_resized, at which point win_resized would be set (again) by the
signal handler.  Shortly thereafter we would clear win_resized even
though we only handled the earlier SIGWINCH but not the latest one.
This chain of events is all avoided if we clear win_resized first.

gdb/ChangeLog:

	* tui/tui-win.c (tui_async_resize_screen): Clear win_resized
	first before resizing the window.
	* tui.c (tui_enable): Likewise.