Prevent DM crash in the PDF sink when src width/height are zero.

dm/DMSrcSink.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "DMSrcSink.h"
 #include "SamplePipeControllers.h"
 #include "SkCommonFlags.h"
@@ skipping 343 matching lines @@
 
     int xPages = ((width - 1) / kLetterWidth) + 1;
     int yPages = ((height - 1) / kLetterHeight) + 1;
 
     for (int y = 0; y < yPages; ++y) {
         for (int x = 0; x < xPages; ++x) {
             int w = SkTMin(kLetterWidth, width - (x * kLetterWidth));
             int h = SkTMin(kLetterHeight, height - (y * kLetterHeight));
             SkCanvas* canvas =
                     doc->beginPage(SkIntToScalar(w), SkIntToScalar(h));
+            if (!canvas) {
+              return "SkDocument::beginPage(w,h) returned NULL";

[djsollen, 2015/04/03 13:35:01]

I think it may be better if we could just produce an empty pdf which would map
to what other backends do.

+            }
             canvas->clipRect(letter);
             canvas->translate(-letter.width() * x, -letter.height() * y);
             Error err = src.draw(canvas);
             if (!err.isEmpty()) {
                 return err;
             }
             doc->endPage();
         }
     }
     doc->close();
@@ skipping 255 matching lines @@
             surfaces.unrefAll();
             return "";
         }
         SkISize size() const override { return fSize; }
         Name name() const override { sk_throw(); return ""; }  // No one should be calling this.
     } proxy(fW, fH, pic, src.size());
     return fSink->draw(proxy, bitmap, stream, log);
 }
 
 }  // namespace DM