fix passing null as Promise's onFullfilled (which is broken in Chrome 32)

src/core/evaluator.js

@@ -253,7 +253,7 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
           var imgData = imageObj.createImageData(/* forceRGBA = */ false);
           self.handler.send('obj', [objId, self.pageIndex, 'Image', imgData],
             [imgData.data.buffer]);
-        }).then(null, function (reason) {
+        }).then(undefined, function (reason) {
           warn('Unable to decode image: ' + reason);
           self.handler.send('obj', [objId, self.pageIndex, 'Image', null]);
         });