Prevent SkBlockMemoryStream::skip from writing to low memory.

Prevent SkBlockMemoryStream::skip from writing to low memory.

SkBlockMemoryStream::read was modeled on SkDynamicMemoryWStream::read.
However, SkDynamicMemoryWStream::read does not allow passing a NULL buffer
and SkBlockMemoryStream::read must correctly handle this case for skips.
In the current code, if a 'skip' crosses a block boundary, 'read' may
attempt to write to low memory.

R=robertphillips@google.com

Committed: https://code.google.com/p/skia/source/detail?r=13671

[bungeman-skia, 2014-03-05 16:13:48]

It appears this wasn't detected before because we happen to currently never call
'skip' on one of these.

[robertphillips, 2014-03-05 16:21:23]

lgtm

[bungeman-skia, 2014-03-05 16:26:17]

Committed patchset #1 manually as r13671.