plugin/{forward,proxy}: check for truncated (#1644)

Check for trunacted in the lookup function as well and use the Match
function here as well.

plugin/proxy/lookup.go

@@ -92,6 +92,11 @@ func (p Proxy) lookup(state request.Request) (*dns.Msg, error) {
 			atomic.AddInt64(&host.Conns, -1)
 
 			if backendErr == nil {
+
+				if !state.Match(reply) {
+					return state.ErrorMessage(dns.RcodeFormatError), nil
+				}
+
 				return reply, nil
 			}