Fix minor problems found by static checking.

Fix minor problems found by static checking.

* frame.c (delete_frame):
* xdisp.c (next_element_from_display_vector):
Avoid uninitialized local.
* image.c (imagemagick_compute_animated_image): Port to C89.
Prefer usual GNU indentation style for loops.
Be more careful about bizarrely large sizes, by using ptrdiff_t
instead of int.