client: endp fsm: allow cancelling a notify event

There is a use-after-free problem if a 'notify' FSM as passed to
osmo_mgcpc_ep_ci_request() deallocates before the notify event has been
dispatched. To avoid that, add API to allow cancelling a notify.

Change-Id: I41687d7f3a808587ab7f7520f46dcc3c29cff92d
diff --git a/include/osmocom/mgcp_client/mgcp_client_endpoint_fsm.h b/include/osmocom/mgcp_client/mgcp_client_endpoint_fsm.h
index d77371a..9ae2039 100644
--- a/include/osmocom/mgcp_client/mgcp_client_endpoint_fsm.h
+++ b/include/osmocom/mgcp_client/mgcp_client_endpoint_fsm.h
@@ -29,6 +29,8 @@
 			      uint32_t event_success, uint32_t event_failure,
 			      void *notify_data);
 
+void osmo_mgcpc_ep_cancel_notify(struct osmo_mgcpc_ep *ep, struct osmo_fsm_inst *notify);
+
 /*! Dispatch a DLCX for the given connection.
  * \param ci  Connection identifier as obtained from osmo_mgcpc_ep_ci_add().
  */