Use some macros to clean up the Snifer decompression code.

For code that's used in more than one place, use macros.  This combines
a bunch of checks into the output macros.

Change-Id: Ic32dce75e1c531fd28bfed180856e230277bfe58
Reviewed-on: https://code.wireshark.org/review/4451
Reviewed-by: Guy Harris <guy@alum.mit.edu>