Free memory in the mock array buffer allocator.

Turns out ASAN and valgrind notice if you leak 0 allocated bytes.

R=jkummerow@chromium.org, jochen@chromium.org

Review URL: https://codereview.chromium.org/306583006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21562 ce2b1a6d-e550-0410-aec6-3dcde31c8c00