net: remove redundant argument check from Dial on udp, unix networks

The net package consists of thin three layers like the follwoing;

- Exposed API, that contains net.Dial, net.DialUDP, net.DialUnix
- Socket and network file descriptor, that contains net.netFD and
  its methods, helper functions such as dialUDP, dialUnix
- Network pollster, that contains net.pollDesc and its methods

This CL removes redundant argument check which is already done by
API layer.

R=golang-dev, dave, bradfitz
CC=golang-dev
https://golang.org/cl/13092043

src/pkg/net/udpsock_posix.go

@@ -170,10 +170,6 @@ func (c *UDPConn) WriteMsgUDP(b, oob []byte, addr *UDPAddr) (n, oobn int, err er
 // which must be "udp", "udp4", or "udp6".  If laddr is not nil, it is
 // used as the local address for the connection.
 func DialUDP(net string, laddr, raddr *UDPAddr) (*UDPConn, error) {
-	return dialUDP(net, laddr, raddr, noDeadline)
-}
-
-func dialUDP(net string, laddr, raddr *UDPAddr, deadline time.Time) (*UDPConn, error) {
 	switch net {
 	case "udp", "udp4", "udp6":
 	default:
@@ -182,6 +178,10 @@ func dialUDP(net string, laddr, raddr *UDPAddr, deadline time.Time) (*UDPConn, e
 	if raddr == nil {
 		return nil, &OpError{"dial", net, nil, errMissingAddress}
 	}
+	return dialUDP(net, laddr, raddr, noDeadline)
+}
+
+func dialUDP(net string, laddr, raddr *UDPAddr, deadline time.Time) (*UDPConn, error) {
 	fd, err := internetSocket(net, laddr, raddr, deadline, syscall.SOCK_DGRAM, 0, "dial", sockaddrToUDP)
 	if err != nil {
 		return nil, err

src/pkg/net/unixsock_posix.go

@@ -247,15 +247,15 @@ func (c *UnixConn) CloseWrite() error {
 // which must be "unix", "unixgram" or "unixpacket".  If laddr is not
 // nil, it is used as the local address for the connection.
 func DialUnix(net string, laddr, raddr *UnixAddr) (*UnixConn, error) {
-	return dialUnix(net, laddr, raddr, noDeadline)
-}
-
-func dialUnix(net string, laddr, raddr *UnixAddr, deadline time.Time) (*UnixConn, error) {
 	switch net {
 	case "unix", "unixgram", "unixpacket":
 	default:
 		return nil, UnknownNetworkError(net)
 	}
+	return dialUnix(net, laddr, raddr, noDeadline)
+}
+
+func dialUnix(net string, laddr, raddr *UnixAddr, deadline time.Time) (*UnixConn, error) {
 	fd, err := unixSocket(net, laddr, raddr, "dial", deadline)
 	if err != nil {
 		return nil, err