silent_call: check return value of trans_alloc() against NULL

We're checking the result of trans_alloc() 6 out of 7 times, so check
it in gsm_silent_call_start() too, for the sake of consistency.

Change-Id: Ie989cd8146d66d9531cf3f3d84f46a2c6fcc2e5c
Fixes: CID#322140
---
M src/libmsc/silent_call.c
1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/src/libmsc/silent_call.c b/src/libmsc/silent_call.c
index 3b95a90..4de12b9 100644
--- a/src/libmsc/silent_call.c
+++ b/src/libmsc/silent_call.c
@@ -140,7 +140,11 @@
 	struct vty *vty)
 {
 	struct gsm_network *net = vsub->vlr->user_ctx;
-	struct gsm_trans *trans = trans_alloc(net, vsub, TRANS_SILENT_CALL, 0, 0);
+	struct gsm_trans *trans;
+
+	trans = trans_alloc(net, vsub, TRANS_SILENT_CALL, 0, 0);
+	if (trans == NULL)
+		return -ENODEV;
 
 	trans->silent_call.ct = *ct;
 	if (traffic_dst_ip) {

