Use WIN_ReleasePtr not WIN_ReleaseWndPtr with WIN_GetPtr.

diff --git a/windows/mdi.c b/windows/mdi.c
index c6e25ac..e6469a8 100644
--- a/windows/mdi.c
+++ b/windows/mdi.c
@@ -190,7 +190,7 @@
         }
         if (win->cbWndExtra < sizeof(MDICLIENTINFO)) WARN( "%x is not an MDI client\n", client );
         else ret = (MDICLIENTINFO *)win->wExtra;
-        WIN_ReleaseWndPtr( win );
+        WIN_ReleasePtr( win );
     }
     return ret;
 }