Avoid segfaults due to image cache being cleared during redisplay

Avoid segfaults due to image cache being cleared during redisplay

* src/xdisp.c (redisplay_internal): Set the
inhibit_clear_image_cache flag of a frame while its windows
are being redisplayed, and reset the flag after the call top
update_frame returns.
* src/image.c (clear_image_cache): Do nothing if the frame's
inhibit_clear_image_cache flag is set.  (Bug#34256)
* src/frame.h (struct frame): New flag inhibit_clear_image_cache.