fix a bug when read the working distance

OxfordExtenderWrapper/ExtenderWrapper.cs

@@ -1114,7 +1114,7 @@ namespace OxfordExtenderWrapper
             ReadMicroscopeColumn();
             if (m_dWorkingDistance != 0)
             {
-                bool ifwdread = true;
+                 ifwdread = true;
             }
             return (float)m_dWorkingDistance;
         }