Free match after criteria initialization

Long description:

This memory leak is associated with matching by a criteria that uses a
regular expression. Without freeing a regex before calling match_init,
it will definitely be lost.

To apply this patch, use:
curl http://cr.i3wm.org/patch/481/raw.patch | git am

@@ -257,6 +257,7 @@ void cmd_criteria_init(I3_CMD) {

     owindow *ow;

     DLOG("Initializing criteria, current_match = %p\n", current_match);

+    match_free(current_match);

     match_init(current_match);

     while (!TAILQ_EMPTY(&owindows)) {

         ow = TAILQ_FIRST(&owindows);