ModbusSerialClient: Initialise new fields.

Not initialising _last_frame_end and _silent_interval breaks test cases.
We'll initialise _last_frame_end to 0.0 (to indicate the last frame is
possibly a *long* time ago, connect resets it) and we set
_silent_interval there since baud rate should not be changing.

pymodbus/client/sync.py

@@ -304,6 +304,8 @@ def __init__(self, method='ascii', **kwargs):
         self.parity   = kwargs.get('parity',   Defaults.Parity)
         self.baudrate = kwargs.get('baudrate', Defaults.Baudrate)
         self.timeout  = kwargs.get('timeout',  Defaults.Timeout)
+        self._last_frame_end = 0.0
+        self._silent_interval = 3.5 * (1 + 8 + 2) / self.baudrate
 
     @staticmethod
     def __implementation(method):
@@ -333,7 +335,6 @@ def connect(self):
             _logger.error(msg)
             self.close()
         self._last_frame_end = time.time()
-        self._silent_interval = 3.5 * (1 + 8 + 2) / self.baudrate
         return self.socket != None
 
     def close(self):