CallWindowProc16 fix

[Stefan Leichter <Stefan.Leichter@camline.com>]

Hello,

this patch fixes the main install program of one of my shareware CD:s 
(Backgammon und andere Brettspiele)

Licence: X11 (for all of my patches, also for my previous two)

ChangeLog
-------------
	Added pointer check for CallWindowProc16 to avoid the debugger

diff -uwr wine-20020411/windows/winproc.c wine/windows/winproc.c
--- wine-20020411/windows/winproc.c     Fri Apr 12 21:44:55 2002
+++ wine/windows/winproc.c      Sat Apr 13 14:49:50 2002
@@ -2563,7 +2563,10 @@
 LRESULT WINAPI CallWindowProc16( WNDPROC16 func, HWND16 hwnd, UINT16 msg,
                                  WPARAM16 wParam, LPARAM lParam )
 {
-    WINDOWPROC *proc = WINPROC_GetPtr( func );
+    WINDOWPROC *proc;
+
+    if (!func) return 0;
+    proc = WINPROC_GetPtr( func );

     if (!proc)
         return WINPROC_CallWndProc16( func, hwnd, msg, wParam, lParam );