src: destroy inspector agent before context

The inspector_agent depends on the context still being accessible
during the destructor execution.

PR-URL: https://github.com/nodejs/node/pull/16472
Fixes: https://github.com/nodejs/node/issues/15558


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>