packetutils: Check for nbody AND sizeof(header)

Change-Id: Ie7a5c3bbb2224bc16604cade5a7241c0a2b5c7a8
Reviewed-on: http://review.couchbase.org/32634
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>