net: fix undetected set and not used error

Fixes an error where the compiler did not spot that the shadowed err
value was set again after being read. That second assignment was lost
as the value was redeclared in an inner scope.

Spotted by Gordon Klass, https://groups.google.com/forum/#!topic/golang-nuts/MdDLbvOjb4o

Change-Id: I28f2da6f98c52afcbb45e17d2b4f36c586598f98
Reviewed-on: https://go-review.googlesource.com/10600Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>

src/net/file_unix.go

@@ -45,7 +45,8 @@ func newFileFD(f *os.File, sa SocketAddr) (*netFD, error) {
 		fd, err = newFD(s, -1, -1, laddr.Network())
 	} else {
 		family := syscall.AF_UNSPEC
-		sotype, err := syscall.GetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_TYPE)
+		var sotype int
+		sotype, err = syscall.GetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_TYPE)
 		if err != nil {
 			closeFunc(s)
 			return nil, os.NewSyscallError("getsockopt", err)