* test/src/image-tests.el (image-test-circular-specs): New file.
* src/image.c (parse_image_spec): Return failure for circular lists.
(valid_image_p): Don't look at odd-numbered list elements expecting to
find a property name.
(image_spec_value): Handle circular lists.
(equal_lists): Introduce.
(search_image_cache): Use `equal_lists' (bug#36403).
