If there are'nt any bpf descriptors to process than dont bother attempting to

retrieve statistic information for them.

Pointed out by:	Pawel Worach < pawel.worach at gmail.com >

usr.bin/netstat/bpf.c

@@ -91,6 +91,8 @@ bpf_stats(char *interface)
 		warn("net.bpf.stats");
 		return;
 	}
+	if (size == 0)
+		return;
 	bd = malloc(size);
 	if (bd == NULL) {
 		warn("malloc failed");