tiff2ps: fix heap-buffer-overflow
http://bugzilla.maptools.org/show_bug.cgi?id=2834
usually the test (i < byte_count) is OK because the byte_count is divisible by samplesperpixel. But if that is not the case, (i + ncomps) < byte_count should be used, or maybe (i + samplesperpixel) <= byte_count