Fixed "reached end of non void..." warnings.

diff --git a/windows/mdi.c b/windows/mdi.c
index 5be8bc3..a519a4a 100644
--- a/windows/mdi.c
+++ b/windows/mdi.c
@@ -633,6 +633,7 @@
 END:
     WIN_ReleaseWndPtr(wndPtr);
     WIN_ReleaseWndPtr(wndPrev);
+    return retvalue;
 }
 
 /* -------------------- MDI client window functions ------------------- */
@@ -1554,6 +1555,7 @@
     retvalue = DefWindowProc16(hwnd, message, wParam, lParam);
 END:
     WIN_ReleaseWndPtr(clientWnd);
+    return retvalue;
 }