Store copy mode search string in pane so search-again command works even

if you exit and reenter copy mode (it doesn't remember the position,
just the search string), suggested by espie@.

window.c

@@ -828,6 +828,7 @@ static void
 window_pane_destroy(struct window_pane *wp)
 {
 	window_pane_reset_mode(wp);
+	free(wp->searchstr);
 
 	if (wp->fd != -1) {
 		bufferevent_free(wp->event);