pfsync: fix memory leak

The recent refactoring to prepare for pfsync over IPv6 introduced a
memory leak.
If we don't have a sync peer configured we return early (without sending
out a packet), but failed to free the newly allocated packet.

Sponsored by:	Rubicon Communications, LLC ("Netgate")

sys/netpfil/pf/if_pfsync.c

@@ -1589,6 +1589,7 @@ pfsync_sendout(int schedswi, int c)
 	    }
 #endif
 	default:
+		m_freem(m);
 		return;
 	}