Handle partial message in ReadDeviceInformationResponse

When a partial message is received for a ReadDeviceInformationResponse,
an exception occurs when calculating the frame size:

 ERROR:asyncio:Exception in callback SerialTransport._read_ready()
 handle: <Handle SerialTransport._read_ready()>
 Traceback (most recent call last):
   File "/usr/lib/python3.9/asyncio/events.py", line 80, in _run
       self._context.run(self._callback, *self._args)
   File "pymodbus/transport/transport_serial.py", line 109, in _read_ready
       self._protocol.data_received(data)
   File "pymodbus/transport/transport.py", line 307, in data_received
       self.datagram_received(data, None)
   File "pymodbus/transport/transport.py", line 328, in datagram_received
       cut = self.callback_data(self.recv_buffer, addr=addr)
   File "pymodbus/client/base.py", line 218, in callback_data
       self.framer.processIncomingPacket(data, self._handle_response, slave=0)
   File "pymodbus/framer/base.py", line 137, in processIncomingPacket
       self.frameProcessIncomingPacket(single, callback, slave, **kwargs)
   File "pymodbus/framer/rtu_framer.py", line 213, in frameProcessIncomingPacket
       if not self.isFrameReady():
   File "pymodbus/framer/rtu_framer.py", line 134, in isFrameReady
       size = self.populateHeader()
   File "pymodbus/framer/rtu_framer.py", line 152, in populateHeader
       size = self.get_expected_response_length(data)
   File "pymodbus/framer/rtu_framer.py", line 320, in get_expected_response_length
       return pdu_class.calculateRtuFrameSize(data)
   File "mei_message.py", line 130, in calculateRtuFrameSize
       _, object_length = struct.unpack(">BB", buffer[size : size + 2])
 struct.error: unpack requires a buffer of 2 bytes

The caller is expecting an IndexError if there isn't enough data in the
packet. So to fix this, catch the struct error and raise an IndexError.

pymodbus/mei_message.py

@@ -125,11 +125,14 @@ def calculateRtuFrameSize(cls, buffer):
         size = 8  # skip the header information
         count = int(buffer[7])
 
-        while count > 0:
-            _, object_length = struct.unpack(">BB", buffer[size : size + 2])
-            size += object_length + 2
-            count -= 1
-        return size + 2
+        try:
+            while count > 0:
+                _, object_length = struct.unpack(">BB", buffer[size : size + 2])
+                size += object_length + 2
+                count -= 1
+            return size + 2
+        except struct.error:
+            raise IndexError
 
     def __init__(self, read_code=None, information=None, **kwargs):
         """Initialize a new instance.