MFC r360231:
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
Sponsored by: RG Nets