Xext: xres: protect against free()ing from non-heap

> ../Xext/xres.c: In function ‘DestroyFragments’:
> ../Xext/xres.c:124:9: warning: ‘free’ of ‘it’ which points to memory on the stack [CWE-590] [-Wanalyzer-free-of-non-heap]
>   124 |         free(it);
>      |         ^~~~~~~~

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

Xext/xres.c

@@ -136,11 +136,13 @@ static void
 DestroyFragments(struct xorg_list *frags)
 {
     FragmentList *it, *tmp;
+    if (!xorg_list_is_empty(frags)) {
         xorg_list_for_each_entry_safe(it, tmp, frags, l) {
             xorg_list_del(&it->l);
             free(it);
         }
     }
+}
 
 /** @brief Constructs a context record for ConstructClientId* functions
            to use */