e1_input: Stop the timer when deleting the signalling link on the TS

Stop the tx_timer when deleting the link on top of that ts. Otherwise
bad things might happen. E.g. when scheduling a write on OML and then
the OML link vanishes...

This is a slight layering violation as there could be more than
one signalling link on the timeslot (at least in theory) so the
queue and the timer should move to the e1inp_sign_link.

openbsc/src/e1_input.c

@@ -428,6 +428,9 @@ void e1inp_sign_link_destroy(struct e1inp_sign_link *link)
 		msgb_free(msg);
 	}
 
+	if (link->ts->type == E1INP_TS_TYPE_SIGN)
+		bsc_del_timer(&link->ts->sign.tx_timer);
+
 	talloc_free(link);
 }