Handle getMemoryBase returning NULL in WriteTask.

DM::WriteTask::fData was changed from a data to a stream in
"SkData to SkStreamAsset to avoid unneeded copying"
https://skia.googlesource.com/skia/+/a4c6094177ebde18c706cbcfbd2013d0a088e0ee
However, DM::WriteTask::draw was not updated to handle this,
resulting in segfaults when trying to write out PDF files.

Review URL: https://codereview.chromium.org/627473002
1 file changed