Add a (#ifdef 0'ed) test provoking a segfault in TIFFReadDirectory().

Add a (#ifdef 0'ed) test provoking a segfault in TIFFReadDirectory().

	* test-images.h (tiff1_test_3), test-loaders.c (main): Add a
	(#ifdef 0'ed) test provoking a segfault in TIFFReadDirectory().

	* io-ico.c (DecodeHeader): Check that pixbuf could be allocated.

	* test-images.h (ico_test_2), test-loaders.c (main): Test the
	previous change.

	* io-wbmp.c (gdk_pixbuf__wbmp_image_load_increment): Don't write
	beyond the end of buffer if width % 8 != 0.

	* io-tga.c (try_preload): Enforce that ctx->hdr->type is one of
	the supported types, otherwise parse_data_for_row () will not make
	any progress, leading to an infinite loop.