Make Isolate::GetHeapStatistics robust against half-initialized isolates.

The (deprecated) non-Isolate GetHeapStatistics contains the same check.

BUG=2591

Review URL: https://codereview.chromium.org/12965013
Patch from Marja Hölttä <marja@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14079 ce2b1a6d-e550-0410-aec6-3dcde31c8c00