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

Review URL: https://codereview.chromium.org/187683003

git-svn-id: http://skia.googlecode.com/svn/trunk@13671 2bbb7eff-a529-9590-31e7-b0007b416f81