coding: fix a copy-paste bug in gsm0503_tch_afs_decode_dtx()

According to 3GPP TS 45.003, section 3.9.2, the convolutional coding
is *not* employed in the case of SID_FIRST.  This is why we're using
sid_first_dummy[] in this function.

The conv[] array is not initialized in the case of AFS_SID_FIRST and
passing it to tch_amr_reassemble() is highly likely a copy-paste bug.
Pass the sid_first_dummy[] instead.

Change-Id: Ic8bfc34ce9d14821fe4e82932325f0c517d443a1
Fixes: CID#272949 "Uninitialized scalar variable"
diff --git a/src/coding/gsm0503_coding.c b/src/coding/gsm0503_coding.c
index 80286f7..286981e 100644
--- a/src/coding/gsm0503_coding.c
+++ b/src/coding/gsm0503_coding.c
@@ -2262,7 +2262,7 @@
 			tch_amr_sid_update_append(sid_first_dummy, 0,
 						  (codec_mode_req) ? codec[*ft]
 						  : codec[id > 0 ? id : 0]);
-			tch_amr_reassemble(tch_data, conv, 39);
+			tch_amr_reassemble(tch_data, sid_first_dummy, 39);
 			len = 5;
 			goto out;
 		case AFS_SID_UPDATE: /* TODO: parse CMI _and_ CMC/CMR (16 + 16 bit) */