pfsync: NULL check before dereference

Move the use of 'sc' to after the NULL check.
It's very unlikely that we'd actually hit this, but Coverity is correct
that it's not a good idea to dereference the pointer and only then NULL
check it.

Reported by:	Coverity (CID 1398362)
MFC after:	1 week
Sponsored by:	Rubicon Communications, LLC ("Netgate")

sys/netpfil/pf/if_pfsync.c

@@ -1735,7 +1735,7 @@ pfsync_defer(struct pf_kstate *st, struct mbuf *m)
 {
 	struct pfsync_softc *sc = V_pfsyncif;
 	struct pfsync_deferral *pd;
-	struct pfsync_bucket *b = pfsync_get_bucket(sc, st);
+	struct pfsync_bucket *b;
 
 	if (m->m_flags & (M_BCAST|M_MCAST))
 		return (0);
@@ -1743,6 +1743,8 @@ pfsync_defer(struct pf_kstate *st, struct mbuf *m)
 	if (sc == NULL)
 		return (0);
 
+	b = pfsync_get_bucket(sc, st);
+
 	PFSYNC_LOCK(sc);
 
 	if (!(sc->sc_ifp->if_drv_flags & IFF_DRV_RUNNING) ||