Ensure that SkMemoryStream does not crash dereferencing fData.

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.

Committed: https://code.google.com/p/skia/source/detail?r=7111

[Leon, 2013-01-09 21:42:28]

Sorry to step on your just added code, Greg, but we actually had a bug in
SkMemoryStream already, which I fixed in https://codereview.appspot.com/6883044/
but have not checked in.

[Humper, 2013-01-09 21:44:51]

On 2013/01/09 21:42:28, scroggo-work wrote:
> Sorry to step on your just added code, Greg, but we actually had a bug in
> SkMemoryStream already, which I fixed in
https://codereview.appspot.com/6883044/
> but have not checked in.

Interesting -- I  think I originally had something closer to what you have here,
but when Mike reviewed the code he suggested that I use the SkSafeRef approach. 
I'll defer to his wisdom on this CL :)

[reed1, 2013-01-09 21:48:34]

lgtm

I think earlier I wanted to avoid creating the emptydata, and then call setData.
This if/else does that.

Also, we clearly had a bug all along in setData, where we didn't check for null.

I guess we need an implementation comment that asserts that fData (the private
field) is *never* null.