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

dm/DMSrcSink.cpp

Index: dm/DMSrcSink.cpp
diff --git a/dm/DMSrcSink.cpp b/dm/DMSrcSink.cpp
index 929ff0556b453de2a5786595fb107e69fdc4febe..3db7582aa151fd4965a55e7e2354c84c14f836f9 100644
--- a/dm/DMSrcSink.cpp
+++ b/dm/DMSrcSink.cpp
@@ -361,6 +361,9 @@ static Error draw_skdocument(const Src& src, SkDocument* doc, SkWStream* dst) {
             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);