MFC r307882: bsnmpd: fix SNMPv3

This change fixes SNMPv3 for bsnmpd in stable/11.
Note this is already fixed in stable/12 and above.

Original commit log:

Fix a regression introduced in SVN r256678 that breaks USM header parsing

Reviewed by:	bz@

(cherry picked from commit 1108712cb5b14130380c93b00f4e9ef2a797a07d)