when calculating a palette with median cut for extraordinarily
large images. CID 185306.
+ - don't check if the unsigned size supplied to
+ im_set_image_file_limits() is negative. CID 185307.
+
[1] The first two build submissions ended up at the end of a ~400
build queue, and seemed to have been cancelled by Coverity. A build
submitted on NYE went through in minutes.