xprops: Free atom_list_from_results() return value with g_free()

atom_list_from_results directly passes through the results->prop
pointer which is g_malloc memory, so we should free the buffer it
returns with g_free.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/786

src/x11/xprops.c

@@ -1092,7 +1092,7 @@ free_value (MetaPropValue *value)
     case META_PROP_VALUE_WINDOW:
       break;
     case META_PROP_VALUE_ATOM_LIST:
-      free (value->v.atom_list.atoms);
+      g_free (value->v.atom_list.atoms);
       break;
     case META_PROP_VALUE_TEXT_PROPERTY:
       free (value->v.str);