Debug a mystery crash bug in script streaming.

This adds a bunch of checks so we hopefully get more detailed information about
what exactly goes wrong.

R=jochen@chromium.org
BUG=

Review URL: https://codereview.chromium.org/619583005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24373 ce2b1a6d-e550-0410-aec6-3dcde31c8c00