Make the NetBIOS dissecting routines all take a pointer to the beginning

Make the NetBIOS dissecting routines all take a pointer to the beginning
of the frame, plus at most one offset from the beginning of the frame,
to make it clearer what the offset is.

Then use that offset in at least some places to do bounds checking.

If a packet has no payload, don't hand it to the SMB dissector.

svn path=/trunk/; revision=1165