ifconfig: Fix invalid free() in ifbridge

parse_vlans() does 's = strdup(str)', then calls strsep(&s, ...), then
attempts to free(s) at the end of the function. For the success case,
this is fine (s is NULL, so it's a trivial memory leak), but in the
error case, we will attempt to free an invalid pointer.

Fix this by storing the original return value from strdup() and freeing
that instead.

MFC after: 3 seconds
Reported by: David Gwynne <dlg@openbsd.org>
Reviewed by: zlei, kevans
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53545