net: fix a bad cast in dnsmsg.go

Incorrect cast was causing panics when
calling String() on dnsMsg with dnsRR_A
answers.

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/6818043

src/pkg/net/dnsmsg.go

@@ -618,7 +618,7 @@ func printStruct(any dnsStruct) string {
 		s += name + "="
 		switch tag {
 		case "ipv4":
-			i := val.(uint32)
+			i := *val.(*uint32)
 			s += IPv4(byte(i>>24), byte(i>>16), byte(i>>8), byte(i)).String()
 		case "ipv6":
 			i := val.([]byte)