Handling of malformed MEI responses

[Tamarinen]

We have a heat pump that manages to confuse pymodbus when asked to identify itself through RTU. When asked for the mandatory information, the response is as follows:

0x01 00x2b 0x0e 0x01 0x01 0x00 0x00 0x01 0x00 0x04 NIBE 0x01 0x08 F1345-60 0x02 0x04 4150 0xab 0x9a

The pump indicate that the MEI response contain only one object, while three are sent, which calculateRtuFrameSize() in mei_message.py doesn't like at all.

I propose this method of verifying the MEI response structure:

@classmethod
def calculateRtuFrameSize(cls, buffer):
    ''' Calculates the size of the message

    :param buffer: A buffer containing the data that have been received.
    :returns: The number of bytes in the response.
    '''
    size = 8 # skip the header information

    while size + 2 < len(buffer):
        _, object_size = struct.unpack('>BB', buffer[size:size+2])
        size += object_size + 2

    return size + 2

Instead of trying to read the specified number of objects, this read objects until the input buffer is exhausted, returning a valid length for a certain class of malformed MEI responses. The same method of parsing the response is used in ReadDeviceInformationResponse.decode().