BUG/MINOR: nameservers: fix error handling in parsing of resolv.conf

BUG/MINOR: nameservers: fix error handling in parsing of resolv.conf

In issue #657, Coverity found a bug in the "nameserver" parser for the
resolv.conf when "parse-resolv-conf" is set. What happens is that if an
unparsable address appears on a "nameserver" line, it will destroy the
previously allocated pointer before reporting the warning, then the next
"nameserver" line will dereference it again and wlil cause a crash. If
the faulty nameserver is the last one, it will only be a memory leak.
Let's just make sure we preserve the pointer when handling the error.
The patch also fixes a typo in the warning.

The bug was introduced in 1.9 with commit 44e609bfa ("MINOR: dns:
Implement `parse-resolv-conf` directive") so the fix needs to be backported
up to 1.9 or 2.0.