Fix segfaults due to using a stale face ID

Fix segfaults due to using a stale face ID

* src/xdisp.c (forget_escape_and_glyphless_faces): New function.
(display_echo_area_1, redisplay_internal): Call it to avoid
reusing stale face IDs for 'escape-glyph' and 'glyphless-char'
faces, which could case a segfault if the frame's face cache was
freed since the last redisplay.  (Bug#21394)
* src/xfaces.c (free_realized_faces): Call forget_escape_and_glyphless_faces.
* src/dispextern.h (forget_escape_and_glyphless_faces): Add prototype.

Copyright-paperwork-exempt: yes