Ensure that SkMemoryStream does not crash dereferencing fData.

SkMemoryStream attempts to dereference fData in multiple places.
Instead of allowing it to be NULL, resulting in a crash, set it
to SkData::NewEmpty().

Add a test for SkStream that will crash when its SkData is set to NULL.

Review URL: https://codereview.appspot.com/7061059

git-svn-id: http://skia.googlecode.com/svn/trunk@7111 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed