debugger: guard against call from non-node context

Fix a segmentation fault when the debug message handler was called from
a context without an associated `node::Environment`.

Fixes: https://github.com/nodejs/node/issues/4261
Fixes: https://github.com/nodejs/node/issues/4322
PR-URL: https://github.com/nodejs/node/pull/4328


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>