fix(plugin): nilness findings (#7556)

Enable nilness linter in govet.

Plugin-by-plugin rationale:

- plugin/transfer: reuse error instead of shadowing it inside the for
  loop by declaring "ret" outside of the loop
- plugin/view: remove redundant err check
- plugin/dnstap: avoid possible nil dereference in error reporting
  path in setup test
- plugin/forward: prevent nil deference or empty-slice dereference on
  error paths in setup test

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

plugin/transfer/notify.go

@@ -39,10 +39,11 @@ func (t *Transfer) Notify(zone string) error {
 
 func sendNotify(c *dns.Client, m *dns.Msg, s string) error {
 	var err error
+	var ret *dns.Msg
 
 	code := dns.RcodeServerFailure
 	for range 3 {
-		ret, _, err := c.Exchange(m, s)
+		ret, _, err = c.Exchange(m, s)
 		if err != nil {
 			continue
 		}