tbf_dl_ass_fsm: Fix missing transition to NONE if DL TBF is nonexistent

If by the time the PktDlAss is to be scheduled by the scheduler the DL
TBF is gone, the FSM will abort the assignment and go back to state
NONE.
However, the transition was missing, ending up in the scheduler trying
to schedule the message unsuccessfuly lots of times per second, clogging
the logs and disrupting normal operation.

Related: OS#5293
Change-Id: I6f421e5ddc9894fee72de1102df35a76cf2f2647

src/tbf_dl_ass_fsm.c

@@ -208,7 +208,9 @@ static struct osmo_fsm_state tbf_dl_ass_fsm_states[] = {
 	},
 	[TBF_DL_ASS_SEND_ASS] = {
 		.in_event_mask = X(TBF_DL_ASS_EV_CREATE_RLCMAC_MSG),
-		.out_state_mask = X(TBF_DL_ASS_WAIT_ACK),
+		.out_state_mask =
+			X(TBF_DL_ASS_WAIT_ACK) |
+			X(TBF_DL_ASS_NONE),
 		.name = "SEND_ASS",
 		.action = st_send_ass,
 	},