coap: fix a bug;

  Bug fix: do initial calls to col_...() *before* the dissection code so
           that proper column info is displayed even if the dissector
           exits early because of an exception, etc.

Change-Id: Idae880aa0addf0484e1df58afa7b5217bfb40489
Reviewed-on: https://code.wireshark.org/review/5154
Reviewed-by: Bill Meier <wmeier@newsguy.com>