xfree86: fix use-after-free issue in checkInput

*dev is the condition of the while loop we're in, reset to NULL after
freeing

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Dave Airlie <airlied@redhat.com>

hw/xfree86/common/xf86Config.c

@@ -2329,6 +2329,7 @@ checkInput(serverLayoutPtr layout, Bool implicit_layout)
 
                 current = dev;
                 free(*dev);
+                *dev = NULL;
 
                 do {
                     *current = *(current + 1);