user32: Verify we got a valid window handle in GetDCEx().

dlls/user32/painting.c

@@ -908,6 +908,8 @@ HDC WINAPI GetDCEx( HWND hwnd, HRGN hrgnClip, DWORD flags )
 
     TRACE("hwnd %p, hrgnClip %p, flags %08x\n", hwnd, hrgnClip, flags);
 
+    if (!IsWindow(hwnd)) return 0;
+
     /* fixup flags */
 
     if (flags & (DCX_WINDOW | DCX_PARENTCLIP)) flags |= DCX_CACHE;

dlls/user32/tests/dce.c

@@ -392,6 +392,23 @@ static void test_invisible_create(void)
     DestroyWindow(hwnd_owndc);
 }
 
+static void test_destroyed_window(void)
+{
+    HDC dc;
+
+    dc = GetDCEx(hwnd_cache, 0, DCX_USESTYLE);
+    ok(!dc, "Got a non-NULL DC (%p) for a destroyed window.\n", dc);
+
+    dc = GetDCEx(hwnd_owndc, 0, DCX_USESTYLE);
+    ok(!dc, "Got a non-NULL DC (%p) for a destroyed window.\n", dc);
+
+    dc = GetDCEx(hwnd_classdc, 0, DCX_USESTYLE);
+    ok(!dc, "Got a non-NULL DC (%p) for a destroyed window.\n", dc);
+
+    dc = GetDCEx(hwnd_classdc2, 0, DCX_USESTYLE);
+    ok(!dc, "Got a non-NULL DC (%p) for a destroyed window.\n", dc);
+}
+
 START_TEST(dce)
 {
     WNDCLASSA cls;
@@ -431,4 +448,11 @@ START_TEST(dce)
     test_dc_visrgn();
     test_begin_paint();
     test_invisible_create();
+
+    DestroyWindow(hwnd_classdc2);
+    DestroyWindow(hwnd_classdc);
+    DestroyWindow(hwnd_owndc);
+    DestroyWindow(hwnd_cache);
+
+    test_destroyed_window();
 }