feat(itho-wpu): check message length

The 5th byte (6th if you include 0x80) contains the message length. To
double check if the message is correct we can check this against the
actual message length.

itho-wpu.py

@@ -89,7 +89,7 @@ class I2CSlave():
             logger.debug(f"Received {b} bytes! Status {s}")
             result = [hex(c) for c in d]
             logger.debug(f"Callback Response: {result}")
-            if self.is_checksum_valid(result):
+            if self.is_checksum_valid(result) and self.is_length_valid(result):
                 self.queue.put(result)
         else:
             logger.debug(f"Received number of bytes was {b}")
@@ -106,6 +106,14 @@ class I2CSlave():
             return False
         return True
 
+    def is_length_valid(self, b):
+        length_in_msg = int(b[4], 0)
+        actual_length = len(b) - 6
+        if length_in_msg != actual_length:
+            logger.debug(f"Length invalid ({length_in_msg} != {actual_length})")
+            return False
+        return True
+
     def close(self):
         self.event_callback.cancel()
         self.pi.bsc_i2c(0)