DTX: fix AMR SID-FIRST detection

Max's note: adjusted test output.

Change-Id: I46477c631bf86345cb757f31d7f2e2935b12adcc
Related: OS#1801
diff --git a/src/codec/gsm690.c b/src/codec/gsm690.c
index c3cb932..b273f0f 100644
--- a/src/codec/gsm690.c
+++ b/src/codec/gsm690.c
@@ -215,7 +215,7 @@
 };
 
 static const uint8_t amr_len_by_ft[16] = {
-	12, 13, 15, 17, 19, 20, 26, 31, 0,  0,  0,  0,  0,  0,  0,  0
+	12, 13, 15, 17, 19, 20, 26, 31, 7,  0,  0,  0,  0,  0,  0,  0
 };
 
 const struct value_string osmo_amr_type_names[] = {
@@ -262,7 +262,7 @@
 	if (rtppayload[1] >> 7)
 		return -ENOTSUP;
 
-	if (payload_len - 2 < amr_len_by_ft[type])
+	if (payload_len < amr_len_by_ft[type])
 		return -ENOTSUP;
 
 	if (ft)