pf: counter argument to pfr_pool_get() may never be NULL

Coverity points out that if counter was NULL when passed to
pfr_pool_get() we could potentially end up dereferencing it.
Happily all users of the function pass a non-NULL pointer. Enforce this
by assertion and remove the pointless NULL check.

Reported by:	Coverity (CID 273309)
MFC after:	1 week
Sponsored by:	Rubicon Communications, LLC ("Netgate")

(cherry picked from commit efc64d02)

sys/netpfil/pf/pf_table.c

@@ -2211,6 +2211,7 @@ pfr_pool_get(struct pfr_ktable *kt, int *pidx, struct pf_addr *counter,
 	int			 idx = -1, use_counter = 0;
 
 	MPASS(pidx != NULL);
+	MPASS(counter != NULL);
 
 	switch (af) {
 	case AF_INET:
@@ -2230,7 +2231,7 @@ pfr_pool_get(struct pfr_ktable *kt, int *pidx, struct pf_addr *counter,
 		return (-1);
 
 	idx = *pidx;
-	if (counter != NULL && idx >= 0)
+	if (idx >= 0)
 		use_counter = 1;
 	if (idx < 0)
 		idx = 0;