network_conf: Resolve Coverity FORWARD_NULL

network_conf: Resolve Coverity FORWARD_NULL

The code compares def->forwarders when deciding to return 0 at a
couple of points, then uses "def->nfwds" as a way to index into
the def->forwarders array.  That reference results in Coverity
complaining that def->forwarders being NULL was checked as part
of an arithmetic OR operation where failure could be any one 5
conditions, but that is not checked when entering the loop to
dereference the array.  Changing the comparisons to use nfwds
will clear the warnings

Signed-off-by: John Ferlan <jferlan@redhat.com>