* image.c: Fix animation cache signature memory leak.

* image.c: Fix animation cache signature memory leak.

Fix some other minor performance problems while we're at it.
(imagemagick_create_cache): Clear just the members that
need clearing.  Don't set update_time, as caller does that now.
(imagemagick_prune_animation_cache, imagemagick_get_animation_cache):
Simplify by using pointer-to-pointer instead of a prev pointer.
(imagemagick_prune_animation_cache): Use make_emacs_time rather
than EMACS_TIME_FROM_DOUBLE, and DestroyString rather than free.
(imagemagick_get_animation_cache): Don't xstrdup the image signature;
it's already a copy.  Free the signature probe unless it's cached.