clean PLC log only if there is something to clean

Otherwise exception happens in Beremiz service.

Traceback (most recent call last):
File "/home/developer/WorkData/PLC/beremiz/beremiz/connectors/PYRO/_init_.py", line 113, in catcher_func
return func(*args, **kwargs)
File "/home/developer/WorkData/PLC/beremiz/beremiz/connectors/PYRO/_init_.py", line 194, in my_local_func
return RemotePLCObjectProxy._getattr_(attrName)(*args, **kwargs)
File "/usr/lib/python2.7/dist-packages/Pyro/core.py", line 381, in _call_
return self._send(self._name, args, kwargs)
File "/usr/lib/python2.7/dist-packages/Pyro/core.py", line 456, in _invokePYRO
return self.adapter.remoteInvocation(name, Pyro.constants.RIF_VarargsAndKeywords, vargs, kargs)
File "/usr/lib/python2.7/dist-packages/Pyro/protocol.py", line 457, in remoteInvocation
return self._remoteInvocation(method, flags, *args)
File "/usr/lib/python2.7/dist-packages/Pyro/protocol.py", line 532, in _remoteInvocation
answer.raiseEx()
File "/usr/lib/python2.7/dist-packages/Pyro/errors.py", line 72, in raiseEx
raise self.excObj
AttributeError: 'PLCObject' object has no attribute '_ResetLogCount'
+--- This exception occured remotely (Pyro) - Remote traceback:
Traceback (most recent call last):
File "/usr/lib/python2.7/dist-packages/Pyro/protocol.py", line 719, in _handleInvocation2
res = obj.Pyro_dyncall(req[1],flags,req[3]) # (method,flags,args)
File "/usr/lib/python2.7/dist-packages/Pyro/core.py", line 108, in Pyro_dyncall
return getattr(self.delegate or self,method) (args,*keywords)
File "/home/developer/WorkData/PLC/beremiz/beremiz/runtime/PLCObject.py", line 101, in ResetLogCount
if self._ResetLogCount is not None:
AttributeError: 'PLCObject' object has no attribute '_ResetLogCount'
+--- End of remote traceback

--- a/controls/LogViewer.py	Wed Apr 05 14:17:02 2017 +0300
+++ b/controls/LogViewer.py	Wed Apr 05 14:24:52 2017 +0300
@@ -552,6 +552,14 @@
 
         self.MessageScrollBar.RefreshThumbPosition()
 
+    def IsPLCLogEmpty(self):
+        empty=True
+        for level, prev in zip(xrange(LogLevelsCount), self.previous_log_count):
+            if prev is not None:
+                empty=False
+                break
+        return empty
+        
     def IsMessagePanelTop(self, message_idx=None):
         if message_idx is None:
             message_idx = self.CurrentMessage
@@ -639,7 +647,7 @@
         event.Skip()
 
     def OnCleanButton(self, event):
-        if self.LogSource is not None:
+        if self.LogSource is not None and not self.IsPLCLogEmpty():
             self.LogSource.ResetLogCount()
         self.ResetLogMessages()
         self.RefreshView()