The length of the pktap header isn't fixed, it's a field in the header.

Don't assume that, once we've dissected the fields in the header we know
about and skipped past them, that the payload follows immediately
afterwards; use the header length from the header to find the payload.

Bug: 10502
Change-Id: I645fe652240e0068d99cf00d19e3705615857d5b
Reviewed-on: https://code.wireshark.org/review/4280
Reviewed-by: Guy Harris <guy@alum.mit.edu>