libc: Shortcut if_indextoname() if index == 0

If the index we're trying to convert is 0 we can avoid a potentially
expensive call to getifaddrs(). No interface has an ifindex of zero, so
we can handle this as an error: set the errno to ENXIO and return NULL.

Submitted by:	Nick Rogers
Reviewed by:	lutz at donnerhacke.de
MFC after:	2 weeks
Sponsored by:	RG Nets
Differential Revision:	https://reviews.freebsd.org/D24524

lib/libc/net/if_indextoname.c

@@ -66,6 +66,11 @@ if_indextoname(unsigned int ifindex, char *ifname)
 	struct ifaddrs *ifaddrs, *ifa;
 	int error = 0;
 
+	if (ifindex == 0) {
+		errno = ENXIO;
+		return(NULL);
+	}
+
 	if (getifaddrs(&ifaddrs) < 0)
 		return(NULL);	/* getifaddrs properly set errno */