virSystemdGetMachineNameByPID: Initialize @reply

I've noticed that variable @reply is not initialized and if
something at the beginning of the function fails, e.g.
virDBusGetSystemBus(), the control jump straight to cleanup label
where dbus_message_unref() is then called over this uninitialized
variable.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/util/virsystemd.c

@@ -181,7 +181,7 @@ char *
 virSystemdGetMachineNameByPID(pid_t pid)
 {
     DBusConnection *conn;
-    DBusMessage *reply;
+    DBusMessage *reply = NULL;
     char *name = NULL, *object = NULL;
 
     if (virDBusIsServiceEnabled("org.freedesktop.machine1") < 0)