Make Isolate::UncheckedCurrent() return NULL if Isolate wasn't yet initialized
to avoid reentrancy when running with the profiler

BUG=none
R=dcarney@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/365863003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22167 ce2b1a6d-e550-0410-aec6-3dcde31c8c00