libpfctl: memory leak fix

We must remember to free the nvlist we create from the kernel's response
to DIOCGETSTATESNV, on every iteration.

Reviewed by:	donner
MFC after:	1 week
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D30957

(cherry picked from commit 0e9f1892ec)

lib/libpfctl/libpfctl.c

@@ -727,6 +727,7 @@ pfctl_get_states(int dev, struct pfctl_states *states)
 			goto out;
 		}
 
+		nvlist_destroy(nvl);
 		nvl = nvlist_unpack(nv.data, nv.len, 0);
 		if (nvl == NULL) {
 			error = EIO;