Fixed erronous CRC handling

If the CRC recieved is not correct in my case my slave got caught in a deadlock, not taking any new requests. This addition fixed that.

pymodbus/framer/rtu_framer.py

@@ -91,7 +91,12 @@ def checkFrame(self):
             data = self._buffer[:frame_size - 2]
             crc = self._buffer[frame_size - 2:frame_size]
             crc_val = (byte2int(crc[0]) << 8) + byte2int(crc[1])
-            return checkCRC(data, crc_val)
+            if checkCRC(data, crc_val):
+                return True
+            else:
+                _logger.debug("CRC invalid, discarding header!!")
+                self.resetFrame()
+                return False
         except (IndexError, KeyError, struct.error):
             return False