netdev-linux: Avoid fiddling with indeterminate data.

netdev-linux: Avoid fiddling with indeterminate data.

If we are using netlink to get stats and get_ifindex() fails, then for
an internal network device we will then swap around a bunch of
indeterminate (uninitialized) data values.  That won't hurt anything--the
caller will still set them to all-1-bits due to the error--but it still
seems wrong.  So this commit avoid it.

Found using Clang (http://clang-analyzer.llvm.org/).