src: add nullptr check for session in DEBUG macro

Currenlty when configuring --debug-http2
/test/parallel/test-http2-getpackedsettings.js will segment fault:

$ out/Debug/node test/parallel/test-http2-getpackedsettings.js
Segmentation fault: 11

This is happening because the settings is created with the Environment in
PackSettings:
Http2Session::Http2Settings settings(env);
This will cause the session to be set to nullptr. When the init
function is later called the expanded DEBUG_HTTP2SESSION macro will
cause the segment fault when the session is dereferenced.

Backport-PR-URL: https://github.com/nodejs/node/pull/20456
PR-URL: https://github.com/nodejs/node/pull/18815


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>