fixes throving error when startingPos object is not defined

bower.json

 {
   "name": "image-mask-cropper",
-  "version": "1.4.1",
+  "version": "1.4.2",
   "homepage": "http://gl.pixers.pl:8080/milosz.sobczak/image-mask-cropper/tree/master",
   "authors": [
     "milosz.sobczak@pixers.pl"

dist/image-mask-cropper.js

@@ -482,6 +482,7 @@
                             diff.y = startMousePos.y - (e.pageY || e.originalEvent.touches[0].pageY);
 
                             throttle(5, function () {
+                                if (startPosCSS === null || diff.x === null || diff.y === null) return;
                                 self._moveCropZoneOrImage (startPosCSS, diff);
                             });
                         }

package.json

 {
   "name": "image-mask-cropper",
   "description": "Jquery library that gives you ability to move images across container using mask as well",
-  "version": "1.4.1",
+  "version": "1.4.2",
   "author": "Miłosz Sobczak <milosz.sobczak@pixers.pl>",
   "devDependencies": {
     "gulp": "3.9.0",

src/image-mask-cropper.js

@@ -482,6 +482,7 @@
                             diff.y = startMousePos.y - (e.pageY || e.originalEvent.touches[0].pageY);
 
                             throttle(5, function () {
+                                if (startPosCSS === null || diff.x === null || diff.y === null) return;
                                 self._moveCropZoneOrImage (startPosCSS, diff);
                             });
                         }