Cygwin/X: Avoid a potential null pointer dereference before input initialization

Avoid a null pointer dereference if WM_MOUSEMOVE occurred before
the input device had been initialized (a timing sensitive bug
occassionally seen during initialization)

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Colin Harrison <colin.harrison@virgin.net>

hw/xwin/winwndproc.c

@@ -717,6 +717,10 @@ winWindowProc (HWND hwnd, UINT message,
       if (s_pScreenPriv == NULL || s_pScreenInfo->fIgnoreInput)
 	break;
 
+      /* We can't do anything without g_pwinPointer */
+      if (g_pwinPointer == NULL)
+        break;
+
       /* Has the mouse pointer crossed screens? */
       if (s_pScreen != miPointerGetScreen(g_pwinPointer))
 	miPointerSetScreen (g_pwinPointer, s_pScreenInfo->dwScreen,