Calling WebCore::SharedBuffer::append(data, 0) on a shared buffer when

its current position is at a segment boundary (4096) ends up adding an
unitialized segment (with uninitialized memory) to the SharedBuffer.
https://bugs.webkit.org/show_bug.cgi?id=99000

Reviewed by Adam Barth.

Source/WebCore:

* platform/SharedBuffer.cpp:
(WebCore::SharedBuffer::append):

LayoutTests:

* mhtml/shared_buffer_bug-expected.txt: Added.
* mhtml/shared_buffer_bug.mht: Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@131315 268f45cc-cd09-0410-ab3c-d52691b4dbfc