Modbus-C south plugin: when reading invalid address, set invalid bit and continue

[ReneWassinkAlliander]

In the present Modbus South plugin, the connection is reset when an invalid address is read. However: this is a valid action from Modbus point-of-view, as it is possible that Modbus addresses are temporarily unavailable. Therefore, better behaviour would be: set the invalid bit of the read value, and continue reading.