Fix pointerIndex out of range exception

Author: rosuH

parallaxbacklayout/src/main/java/com/github/anzewei/parallaxbacklayout/ViewDragHelper.java

@@ -1226,6 +1226,9 @@ public void processTouchEvent(MotionEvent ev) {
             case MotionEvent.ACTION_MOVE: {
                 if (mDragState == STATE_DRAGGING) {
                     final int index = ev.findPointerIndex(mActivePointerId);
+                    if (ev.getPointerCount() <= index || index < 0){
+                        return;
+                    }
                     final float x = ev.getX( index);
                     final float y = ev.getY(index);
                     final int idx = (int) (x - mLastMotionX[mActivePointerId]);
@@ -1580,4 +1583,4 @@ private int getEdgeTouched(int x, int y) {
 
         return result;
     }
-}
+}