mountd(8): parsecred(): Re-order operations for clarity

No functional change (intended).

Reviewed by: rmacklem (older version)
Approved by: markj (mentor)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47015

(cherry picked from commit a20d50245f280be404cb8e3b5c9d570ded9594b9)