[NTUSER] Fix 'Trying to link windows to itself' (#4478) CORE-18132

Fix 'Trying to link windows to itself' on DestroyWindow. Patch by I_Kill_Bugs.
This fixes a potential BSOD 0x50 observable in the app Localizer Editor"

win32ss/user/ntuser/winpos.c

@@ -1393,8 +1393,34 @@ WinPosDoOwnedPopups(PWND Window, HWND hWndInsertAfter)
 
                if (List[i] == Owner)
                {
-                  if (i > 0) hWndInsertAfter = List[i-1];
+                  /* We found its Owner, so we must handle it here. */
-                  else hWndInsertAfter = topmost ? HWND_TOPMOST : HWND_TOP;
+                  if (i > 0)
+                  {
+                     if (List[i - 1] != UserHMGetHandle(Window))
+                     {
+                        /*
+                         * If the popup to be inserted is not already just
+                         * before the Owner, insert it there. The modified
+                         * hWndInsertAfter will be handled below.
+                         *
+                         * (NOTE: Do not allow hWndInsertAfter to become equal
+                         * to the popup's window handle, as this would cause
+                         * the popup to link to itself).
+                         */
+                        hWndInsertAfter = List[i - 1];
+                     }
+                     else
+                     {
+                        /* If the popup to be inserted is already
+                         * before the Owner, we are done. */
+                        ExFreePoolWithTag(List, USERTAG_WINDOWLIST);
+                        return hWndInsertAfter;
+                     }
+                  }
+                  else
+                  {
+                     hWndInsertAfter = topmost ? HWND_TOPMOST : HWND_TOP;
+                  }
                   break;
                }