tbf_dl_fsm: Fix assert hit due to EV_MAX_N3105 received in ST_RELEASING

That code path was incorrectly removed a few commits back, re-add it.

Fixes: 40a297f3b0c8e1670d46a4974750dd3335bc7885
Change-Id: I27e61dc4b3396360327dcd377d171daa30684d23
diff --git a/src/tbf_dl_fsm.c b/src/tbf_dl_fsm.c
index 8d95d14..917c4b1 100644
--- a/src/tbf_dl_fsm.c
+++ b/src/tbf_dl_fsm.c
@@ -305,6 +305,14 @@
 		 * already ACKED since we left ST_FINISHED. This happens due to
 		 * fn-advance scheduling several DL blocks in advance. */
 		break;
+	case TBF_EV_MAX_N3105:
+		/* Triggered potentially by a poll timeout of PKT UL/DL TBF ASS.
+		 * Reached N3105, the MS is not responding, so stop attempting
+		 * using the TBF for DL assignment and change to ST_RELEASING in
+		 * order to simply wait until resources can be reused (see
+		 * st_releasing_on_enter()). */
+		tbf_dl_fsm_state_chg(fi, TBF_ST_RELEASING);
+		break;
 	default:
 		OSMO_ASSERT(0);
 	}