strptime: fix bug introduced in r272273.

Reported by:	portmgr (antoine)
Fix by:		Andrey Chernov, David Carlier
PR:		137307 (follow up)


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272387 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f