Battery service payload format check

[jmlich]

I have seen some segfaults because of invalid battery service data.
Similar checks are also in other services. For example:

harbour-amazfish/daemon/src/services/infinitimemotionservice.cpp

Lines 33 to 37 in d0508a6

	qDebug() << Q_FUNC_INFO << characteristic << value.toHex();
	if (characteristic == UUID_CHARACTERISTIC_MOTION_STEPS) {
	if (value.length() == 4) {
	m_steps = TypeConversion::toUint32(value[0], value[1], value[2], value[3]);

Test failed due to #468