[gtk/fix-win32-empty-clipboard] gdkclipboard-win32.c: Fix call to gdk_clipboard_claim_remote()

[Chun-wei Fan <fanchunwei src gnome org>]

commit 8519ab56f51eeba585fe5b2d3f2ab3c322904621
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Thu Mar 31 10:36:45 2022 +0800

    gdkclipboard-win32.c: Fix call to gdk_clipboard_claim_remote()
    
    The call to gdk_win32_clipboard_request_contentformats() can return NULL even
    without an error condition being hit (such as when the system clipboard is
    empty), so check whether the returned GdkContentFormat pointer is not NULL
    before calling gdk_clipboard_claim_remote(), which expects it to be not NULL,
    otherwise we face a warning from that funtion and the subsequent
    g_object_unref().
    
    This at least partially fixes issue #4796.

 gdk/win32/gdkclipboard-win32.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)
---
diff --git a/gdk/win32/gdkclipboard-win32.c b/gdk/win32/gdkclipboard-win32.c
index 36d56b8b53..ce80019e56 100644
--- a/gdk/win32/gdkclipboard-win32.c
+++ b/gdk/win32/gdkclipboard-win32.c
@@ -142,9 +142,13 @@ gdk_win32_clipboard_claim_remote (GdkWin32Clipboard *cb)
   cb->sequence_number = -1;
 
   formats = gdk_win32_clipboard_request_contentformats (cb);
-  gdk_clipboard_claim_remote (GDK_CLIPBOARD (cb), formats);
-  gdk_content_formats_unref (formats);
-  cb->sequence_number = GetClipboardSequenceNumber ();
+
+  if (formats != NULL)
+    {
+      gdk_clipboard_claim_remote (GDK_CLIPBOARD (cb), formats);
+      gdk_content_formats_unref (formats);
+      cb->sequence_number = GetClipboardSequenceNumber ();
+    }
 }
 
 static void