fix for Issue #46

Fix for null pointer exception when trying to close the previous opened item, so only one item can be open at a time. This exception occurred when trying to open the last item in the listview.

Example code : 

        listView.setSwipeListViewListener(new BaseSwipeListViewListener() {
            int lastKnownPosition = -1;

            @Override
            public void onOpened(int position, boolean toRight) {
                if (position != lastKnownPosition) listView.closeAnimate(lastKnownPosition);
                lastKnownPosition = position;
            }

        });