V8 API: Add a missing NULL check into Isolate::GetCurrentContext().

There is a missing NULL check for: "internal_isolate->context() != NULL".
Right now before calling this method one should call v8::Context::InContext()
first to perform this check, otherwise we may crash. But this static method
will do this check on the current isolate, which may not be the same as a given one.

BUG=249655
R=yurys@chromium.org,mvstanton@chromium.org

Review URL: https://codereview.chromium.org/16943006

Patch from Andrey Adaykin <aandrey@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15266 ce2b1a6d-e550-0410-aec6-3dcde31c8c00