skip the extra field in the gzip header
it consists of two bytes xsize + xsize bytes of data

use an unsigned type for xsize

fail gracefully if the field is present but truncated

tvb_length_remaining > tvb_captured_length_remaining

Change-Id: I7f5138743c2d88abdd4f5f18d3c0292612ddb559
Reviewed-on: https://code.wireshark.org/review/5654
Reviewed-by: Anders Broman <a.broman58@gmail.com>