Don't add the protocol to the tree if heuristics fail.
Make sure that we have enough bytes to perform the heuristics.
If the magic number is wrong, don't go on to retrieve the ifd offset.
file-tiff.c:840:1: warning: no previous prototype for function 'proto_register_tiff' [-Wmissing-prototypes]
file-tiff.c:1143:1: warning: no previous prototype for function 'proto_reg_handoff_tiff' [-Wmissing-prototypes]