reset assignment codec on start_assignment

Make sure no stray values from previous assignment linger. When starting
Assignment, by definition codec_filter.assignment must be empty.

Change-Id: I58c25bbea9523b212934e9dad551e94bc01d6bba
diff --git a/src/libmsc/msc_a.c b/src/libmsc/msc_a.c
index a6db397..d63e8bd 100644
--- a/src/libmsc/msc_a.c
+++ b/src/libmsc/msc_a.c
@@ -1854,6 +1854,7 @@
 	msc_a->cc.active_trans = cc_trans;
 
 	cc_trans->cc.mncc_release_sent = false;
+	cc_trans->cc.codecs.assignment = (struct sdp_audio_codec){};
 
 	OSMO_ASSERT(cc_trans && cc_trans->type == TRANS_CC);
 	cl = msc_a_ensure_call_leg(msc_a, cc_trans);