net: fix possible nil pointer dereference on ReadFrom for windows

Fixes #10516.

Change-Id: Ia93f53d4e752bbcca6112bc75f6c3dbe30b90dac
Reviewed-on: https://go-review.googlesource.com/9192Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/fd_windows.go

@@ -481,8 +481,12 @@ func (fd *netFD) readFrom(buf []byte) (int, syscall.Sockaddr, error) {
 		o.rsan = int32(unsafe.Sizeof(*o.rsa))
 		return syscall.WSARecvFrom(o.fd.sysfd, &o.buf, 1, &o.qty, &o.flags, o.rsa, &o.rsan, &o.o, nil)
 	})
+	err = fd.eofError(n, err)
+	if err != nil {
+		return n, nil, err
+	}
 	sa, _ := o.rsa.Sockaddr()
-	return n, sa, fd.eofError(n, err)
+	return n, sa, err
 }
 
 func (fd *netFD) Write(buf []byte) (int, error) {