inspector: fix request path nullptr dereference

Fix a nullptr dereference when an invalid path is requested.

Regression introduced in commit 69fc85dc ("inspector: generate UUID for
debug targets"), caught by Coverity.

PR-URL: https://github.com/nodejs/node/pull/9184


Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: James M Snell <jasnell@gmail.com>