Bluetooth: Audio: Add conn check for CSIS client notify handlers

Add a NULL check for the conn pointer in the CSIS client
notify handlers.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>

Author: Thalley

subsys/bluetooth/audio/csis_client.c

@@ -186,6 +186,10 @@ static uint8_t sirk_notify_func(struct bt_conn *conn,
 		return BT_GATT_ITER_STOP;
 	}
 
+	if (conn == NULL) {
+		return BT_GATT_ITER_CONTINUE;
+	}
+
 	csis_inst = lookup_instance_by_handle(conn, handle);
 
 	if (csis_inst != NULL) {
@@ -254,6 +258,10 @@ static uint8_t size_notify_func(struct bt_conn *conn,
 		return BT_GATT_ITER_STOP;
 	}
 
+	if (conn == NULL) {
+		return BT_GATT_ITER_CONTINUE;
+	}
+
 	csis_inst = lookup_instance_by_handle(conn, handle);
 
 	if (csis_inst != NULL) {
@@ -297,6 +305,10 @@ static uint8_t lock_notify_func(struct bt_conn *conn,
 		return BT_GATT_ITER_STOP;
 	}
 
+	if (conn == NULL) {
+		return BT_GATT_ITER_CONTINUE;
+	}
+
 	csis_inst = lookup_instance_by_handle(conn, handle);
 
 	if (csis_inst != NULL) {