silence error messages about HANDOVER_END not permitted

The MSC_A_EV_HANDOVER_END exists as parent term event for the msc_ho_fsm, but
it is not actually required as functional event, since all cleanup is handled
in msc_ho_fsm_cleanup().

That's why I never bothered to add the event to msc_a_fsm, but of course that
means we get an error message after each (successful and unsuccessful)
handover, that the MSC_A_EV_HANDOVER_END is not permitted.

Allow the event and ignore it to silence the error message.
Explain in a comment.

Change-Id: Ie8dc0c0a631b7da43111f329562007766a21b134
diff --git a/src/libmsc/msc_a.c b/src/libmsc/msc_a.c
index 77dd43d..5137e20 100644
--- a/src/libmsc/msc_a.c
+++ b/src/libmsc/msc_a.c
@@ -646,6 +646,12 @@
 		msc_ho_start(msc_a, (struct ran_handover_required*)data);
 		return;
 
+	case MSC_A_EV_HANDOVER_END:
+		/* Termination event of the msc_ho_fsm. No action needed, it's all done in the msc_ho_fsm cleanup. This
+		 * event only exists because osmo_fsm_inst_alloc_child() requires a parent term event; and maybe
+		 * interesting for logging. */
+		return;
+
 	case MSC_A_EV_MO_CLOSE:
 	case MSC_A_EV_CN_CLOSE:
 	case MSC_A_EV_UNUSED:
@@ -756,6 +762,10 @@
 		/* RTP streams cleaned up above */
 		return;
 
+	case MSC_A_EV_HANDOVER_END:
+		/* msc_ho_fsm does cleanup. */
+		return;
+
 	default:
 		OSMO_ASSERT(false);
 	}
@@ -921,6 +931,7 @@
 			| S(MSC_EV_CALL_LEG_TERM)
 			| S(MSC_MNCC_EV_CALL_ENDED)
 			| S(MSC_A_EV_HANDOVER_REQUIRED)
+			| S(MSC_A_EV_HANDOVER_END)
 			,
 		.out_state_mask = 0
 			| S(MSC_A_ST_RELEASING)
@@ -935,6 +946,7 @@
 			| S(MSC_A_EV_UNUSED)
 			| S(MSC_EV_CALL_LEG_TERM)
 			| S(MSC_MNCC_EV_CALL_ENDED)
+			| S(MSC_A_EV_HANDOVER_END)
 			,
 		.out_state_mask = 0
 			| S(MSC_A_ST_RELEASED)