catch oldValue in setValue directly at the beginning (homebridge#687)

Author: Apollon77

lib/Characteristic.js

@@ -315,6 +315,8 @@ Characteristic.prototype.setValue = function(newValue, callback, context, connec
 
   newValue = this.validateValue(newValue); //validateValue returns a value that has be cooerced into a valid value.
 
+  var oldValue = this.value;
+
   if (this.listeners('set').length > 0) {
 
     // allow a listener to handle the setting of this value, and wait for completion
@@ -328,7 +330,6 @@ Characteristic.prototype.setValue = function(newValue, callback, context, connec
         if (newValue === undefined || newValue === null)
           newValue = this.getDefaultValue();
         // setting the value was a success; so we can cache it now
-        var oldValue = this.value;
         this.value = newValue;
         if (callback) callback();
 
@@ -343,7 +344,6 @@ Characteristic.prototype.setValue = function(newValue, callback, context, connec
     if (newValue === undefined || newValue === null)
       newValue = this.getDefaultValue();
     // no one is listening to the 'set' event, so just assign the value blindly
-    var oldValue = this.value;
     this.value = newValue;
     if (callback) callback();