TYPE_CHECK in v8.h should assign to volatile qualified null-pointer.

Unless the pointer is volatile qualified, Clang will warn that LLVM
removes the assignment during optimization. This is not a problem as
that code should never execute, but the warning is treated as an error
when building Chromium, and thus stops the build.

Code review URL: http://codereview.chromium.org/3005010/show

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5093 ce2b1a6d-e550-0410-aec6-3dcde31c8c00