MT#61856 call_flags: `call_ng_flags_str_pair_ht()` might return error

If in the future `call_ng_flags_str_pair_ht()` starts returning
an error as string, pass it up to the caller's caller, in this case
`call_ng_flags_sdp_attr_helper_subst()`.

Change-Id: I59cd988bc30f62c2de159e614a195f8c1182814c

daemon/call_flags.c

@@ -637,7 +637,7 @@ static const char *call_ng_flags_sdp_attr_helper_remove(str *s, unsigned int idx
 static const char *call_ng_flags_sdp_attr_helper_subst(str *s, unsigned int idx, helper_arg arg) {
 	struct sdp_manipulations *sm = call_ng_flags_sdp_attr_helper(s, arg.flags);
 	if (sm)
-		call_ng_flags_str_pair_ht(s, idx, &sm->subst_commands);
+		return call_ng_flags_str_pair_ht(s, idx, &sm->subst_commands);
 	return NULL;
 }