fixed local DoS when UnregisterHandler was called for a not existing handler

Any user with DBUS access could cause a SEGFAULT in tcmu-runner by
running something like this:

dbus-send --system --print-reply --dest=org.kernel.TCMUService1 /org/kernel/TCMUService1/HandlerManager1 org.kernel.TCMUService1.HandlerManager1.UnregisterHandler string:123
static int dbus_handler_handle_cmd(struct tcmu_device *dev,
				   struct tcmulib_cmd *cmd)
{
	abort();
}

static int dbus_handler_handle_cmd(struct tcmu_device *dev,
				   struct tcmulib_cmd *cmd)
{
	abort();
}
