When initializing the LZW state, watch out for a completely bogus

	* plug-ins/common/gifload.c: When initializing the LZW state,
	watch out for a completely bogus input_code_size [based on fix
	by Raphael Quinet]  Also, fix a stupid old bug when clearing the
	code table between subimages.  (Enables us to deal better with errors
	when the stream is corrupted pretty early in a subimage.) [me]