/* read gif signature */
bytestream_get_buffer(&s->bytestream, sig, 6);
if (memcmp(sig, gif87a_sig, 6) != 0 &&
memcmp(sig, gif89a_sig, 6) != 0)
/* read gif signature */
bytestream_get_buffer(&s->bytestream, sig, 6);
if (memcmp(sig, gif87a_sig, 6) != 0 &&
memcmp(sig, gif89a_sig, 6) != 0)