check for null-device pixels in onWritePixels, so we don't crash

https://code.google.com/p/chromium/issues/detail?id=352616

Review URL: https://codereview.chromium.org/196223013

git-svn-id: http://skia.googlecode.com/svn/trunk@13802 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed