icmp: fix ListenPacket shadowing bug.

Change-Id: I78c9a2e06da5269140212996842ac2e885d2746c
Reviewed-on: https://go-review.googlesource.com/10561
Reviewed-by: David Symonds <dsymonds@golang.org>

icmp/listen_posix.go

@@ -57,7 +57,7 @@ func ListenPacket(network, address string) (*PacketConn, error) {
 			proto = iana.ProtocolIPv6ICMP
 		}
 	}
-	var err error
+	var cerr error
 	var c net.PacketConn
 	switch family {
 	case syscall.AF_INET, syscall.AF_INET6:
@@ -80,12 +80,12 @@ func ListenPacket(network, address string) (*PacketConn, error) {
 		}
 		f := os.NewFile(uintptr(s), "datagram-oriented icmp")
 		defer f.Close()
-		c, err = net.FilePacketConn(f)
+		c, cerr = net.FilePacketConn(f)
 	default:
-		c, err = net.ListenPacket(network, address)
+		c, cerr = net.ListenPacket(network, address)
 	}
-	if err != nil {
-		return nil, err
+	if cerr != nil {
+		return nil, cerr
 	}
 	switch proto {
 	case iana.ProtocolICMP: