http2: safer Http2Session destructor

It's hypothetically (and with certain V8 flags) possible for the session
to be garbage collected before all the streams are. In that case, trying
to remove the stream from the session will lead to a segfault due to
attempting to access no longer valid memory. Fix this by unsetting the
session on any streams still around when destroying.

PR-URL: https://github.com/nodejs/node/pull/21194


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>