DImuthuUpe#619: fixed NPE while getPageAtOffset.

Author: Yuriy Bereguliak

android-pdf-viewer/src/main/java/com/github/barteksc/pdfviewer/DragPinchManager.java

@@ -83,6 +83,9 @@ public boolean onSingleTapConfirmed(MotionEvent e) {
 
     private boolean checkLinkTapped(float x, float y) {
         PdfFile pdfFile = pdfView.pdfFile;
+        if (pdfFile == null) {
+            return false;
+        }
         float mappedX = -pdfView.getCurrentXOffset() + x;
         float mappedY = -pdfView.getCurrentYOffset() + y;
         int page = pdfFile.getPageAtOffset(pdfView.isSwipeVertical() ? mappedY : mappedX, pdfView.getZoom());