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/view/setup.go

@@ -52,9 +52,6 @@ func parse(c *caddy.Controller) (*View, error) {
 					return v, err
 				}
 				v.progs = append(v.progs, prog)
-				if err != nil {
-					return nil, err
-				}
 				continue
 			default:
 				return nil, c.Errf("unknown property '%s'", c.Val())