vm: fix crash on fatal error in debug context

Ensure that the debug context has an Environment assigned in case
a fatal error is raised.

The fatal exception handler in node.cc is not equipped to deal with
contexts that don't have one and can't easily be taught that due to
a deficiency in the V8 API: there is no way for the embedder to tell
if the data index is in use.

Fixes: https://github.com/iojs/io.js/issues/1190
PR-URL: https://github.com/iojs/io.js/pull/1229


Reviewed-By: Fedor Indutny <fedor@indutny.com>