* test-images.h: Add valid_ras_test and invalid_bmp_1.

	* test-loaders.c (main): Add random tests for ras and pnm and an
	invalid bmp which was a crasher until my 2002-03-12 commit.

	* io-pnm.c (gdk_pixbuf__pnm_image_load_increment): Don't unref the
	pixbuf on errors.

	* io-ras.c: Handle some errors.