pf: Do not allow negative ps_len in DIOCGETSTATES

Userspace may pass a negative ps_len value to us, which causes an
assertion failure in malloc().
Treat negative values as zero, i.e. return the required size.

Reported-by:	syzbot+53370d9d0358ee2a059a@syzkaller.appspotmail.com
Reviewed by:	lutz at donnerhacke.de
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D24447

sys/netpfil/pf/pf_ioctl.c

@@ -2163,7 +2163,7 @@ pfioctl(struct cdev *dev, u_long cmd, caddr_t addr, int flags, struct thread *td
 		struct pfsync_state	*pstore, *p;
 		int i, nr;
 
-		if (ps->ps_len == 0) {
+		if (ps->ps_len <= 0) {
 			nr = uma_zone_get_cur(V_pf_state_z);
 			ps->ps_len = sizeof(struct pfsync_state) * nr;
 			break;