pdfviewer: more simple fixed to prevent crashes

experimental/PdfViewer/SkPdfRenderer.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkCanvas.h"
 #include "SkDevice.h"
 #include "SkForceLinking.h"
@@ skipping 1931 matching lines @@
         return false;
     }
 
     if (page < 0 || page >= pages()) {
         return false;
     }
 
     PdfContext pdfContext(fPdfDoc);
 
     SkPdfNativeTokenizer* tokenizer = fPdfDoc->tokenizerOfPage(page, pdfContext.fTmpPageAllocator);
+    if (!tokenizer) {
+        // TODO(edisonn): report/warning/debug
+        return false;
+    }
 
     pdfContext.fOriginalMatrix = SkMatrix::I();
     pdfContext.fGraphicsState.fResources = fPdfDoc->pageResources(page);
 
     gPdfContext = &pdfContext;
 
     // TODO(edisonn): get matrix stuff right.
     SkScalar z = SkIntToScalar(0);
     SkScalar w = dst.width();
     SkScalar h = dst.height();
@@ skipping 74 matching lines @@
 }
 
 SkRect SkPdfRenderer::MediaBox(int page) const {
     SkASSERT(fPdfDoc);
     return fPdfDoc->MediaBox(page);
 }
 
 size_t SkPdfRenderer::bytesUsed() const {
     return fPdfDoc ? fPdfDoc->bytesUsed() : 0;
 }