clang: Initialize rc with zero.

In the error case we are assigning errno values
but for the success case we have not assigned
anything to the rc variable.
diff --git a/openbsc/src/silent_call.c b/openbsc/src/silent_call.c
index 8bd16d2..29ade00 100644
--- a/openbsc/src/silent_call.c
+++ b/openbsc/src/silent_call.c
@@ -41,7 +41,7 @@
 {
 	struct gsm_subscriber_connection *conn = _conn;
 	struct scall_signal_data sigdata;
-	int rc;
+	int rc = 0;
 
 	if (hooknum != GSM_HOOK_RR_PAGING)
 		return -EINVAL;