use the supplied handle in IO::Handle::blocking() on Win32

The refactor in 375ed12 meant that fd was initialized to -1,
rather than PerlIO_fileno(f) on Win32.

fixes #17455

dist/IO/IO.xs

@@ -72,8 +72,6 @@ static int
 io_blocking(pTHX_ InputStream f, int block)
 {
     int fd = -1;
-#if defined(HAS_FCNTL)
-    int RETVAL;
     if (!f) {
 	errno = EBADF;
 	return -1;
@@ -83,7 +81,8 @@ io_blocking(pTHX_ InputStream f, int block)
       errno = EBADF;
       return -1;
     }
-    RETVAL = fcntl(fd, F_GETFL, 0);
+#if defined(HAS_FCNTL)
+    int RETVAL = fcntl(fd, F_GETFL, 0);
     if (RETVAL >= 0) {
 	int mode = RETVAL;
 	int newmode = mode;
@@ -129,8 +128,8 @@ io_blocking(pTHX_ InputStream f, int block)
     if (block >= 0) {
 	unsigned long flags = !block;
 	/* ioctl claims to take char* but really needs a u_long sized buffer */
-	const int ret = ioctl(fd, FIONBIO, (char*)&flags);
-	if (ret != 0)
+
+	if (ioctl(fd, FIONBIO, (char*)&flags) != 0)
 	    return -1;
 	/* Win32 has no way to get the current blocking status of a socket.
 	 * However, we don't want to just return undef, because there's no way

dist/IO/t/io_xs.t

@@ -11,7 +11,7 @@ BEGIN {
     }
 }
 
-use Test::More tests => 11;
+use Test::More tests => 13;
 use IO::File;
 use IO::Seekable;
 
@@ -60,7 +60,6 @@ SKIP:
 	or diag "sync(): ", $!;
 }
 
-
 SKIP: {
     # gh 6799
     #
@@ -86,3 +85,22 @@ SKIP: {
     ok(!$fh->error, "no spurious error reported by error()");
     close $fh;
 }
+
+SKIP:
+{
+    # gh #17455
+    # the IO::Handle::blocking() implementation on Win32 was always using
+    # a fd of -1.
+    # A typical caller would be creating the socket using IO::Socket
+    # which blesses the result into that class and that provides its own
+    # blocking implementation and doesn't have this problem.
+    #
+    # The issue here was Win32 specific, but nothing else appears to test
+    # this implementation.
+    use Socket;
+    ok(socket(my $sock, PF_INET, SOCK_STREAM, getprotobyname('tcp')),
+       "create a socket to make non-blocking")
+       or skip "couldn't create the socket: $!", 1;
+    ok(defined IO::Handle::blocking($sock, 0),
+       "successfully make socket non-blocking");
+}