pfsync: NULL check sc before using it

In pfsync_defer() we must wait to lock sc until we've ensured it's not
NULL.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D33240

(cherry picked from commit 27bd812c5c92af9e7acdbf100bd98166a717bb76)