net: make newLocalPacketListener handle network argument correcly

Change-Id: I41691134770d01805c19c0f84f8828b00b85de0c
Reviewed-on: https://go-review.googlesource.com/22058
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/mockserver_test.go

@@ -336,13 +336,21 @@ func timeoutTransmitter(c Conn, d, min, max time.Duration, ch chan<- error) {
 
 func newLocalPacketListener(network string) (PacketConn, error) {
 	switch network {
-	case "udp", "udp4", "udp6":
+	case "udp":
 		if supportsIPv4 {
 			return ListenPacket("udp4", "127.0.0.1:0")
 		}
 		if supportsIPv6 {
 			return ListenPacket("udp6", "[::1]:0")
 		}
+	case "udp4":
+		if supportsIPv4 {
+			return ListenPacket("udp4", "127.0.0.1:0")
+		}
+	case "udp6":
+		if supportsIPv6 {
+			return ListenPacket("udp6", "[::1]:0")
+		}
 	case "unixgram":
 		return ListenPacket(network, testUnixAddr())
 	}