Avoid calling bpf_filter() with len == 0, which causes a change in semantics

(it treats the buffer pointer as an mbuf pointer) and subsequent panic.

MFC after:	3 days
Reported by:	Tony Hariman <tony@cbn.net.id>

sys/netgraph/ng_bpf.c

@@ -403,6 +403,9 @@ ng_bpf_rcvdata(hook_p hook, item_p item)
 		data = mtod(m, u_char *);
 
 	/* Run packet through filter */
+	if (totlen == 0)
+		len = 0;	/* don't call bpf_filter() with totlen == 0! */
+	else
 		len = bpf_filter(hip->prog->bpf_prog, data, totlen, totlen);
 	if (needfree)
 		FREE(data, M_NETGRAPH_BPF);