osmo_bsc_bssap: actually check for lchan

The function bssmap_handle_cipher_mode() suggests to check if an lchan
is actually present when it gets called, but it only checks for conn.
This might lead to a segfault later in the execution path.

Change-Id: I3103ec89cd6dce1a11ea8e9f8187373e4114e852

src/osmo-bsc/osmo_bsc_bssap.c

@@ -487,7 +487,7 @@ static int bssmap_handle_cipher_mode(struct gsm_subscriber_connection *conn,
 	uint8_t enc_bits_msc;
 	int chosen_cipher;
 
-	if (!conn) {
+	if (!conn || !conn->lchan) {
 		LOGP(DMSC, LOGL_ERROR, "No lchan/msc_data in cipher mode command.\n");
 		return -1;
 	}