Remove session use-after free

[shinrich]

I had added these client_vc checks to try and address stale references to client_vc pointers that we were seeing. However referencing the session object after calling do_io_close is dangerous because the session object may have been freed on return. @bneradt caught this as a use-after-free when working with an ASAN build and the traffic-dump plugin in our prod sym environment.

My current theory on the stale client_vc is that the netvc is closing due to EOS or an error while the read/write_vio's have the continuation set to 0. In that case the SM/tunnel/session will not be notified that the netvc has been deleted. I added a Warning message so we can look for that case in our logs. Thought about adding an assert, but I figured the Warning would be less invasive. It may be ok to have an unattached netvc in some cases.

I also removed one set of do_io_read null's that I don't think are necessary.

[shinrich]

[approve ci debian]

[ywkaras]

Have you or @bneradt retested to make sure this is really a fix?

[shinrich]

Yes, @bneradt reran ASAN with this change.

[shinrich]

There are better ways of doing this by reconsidering the "fix" that caused this use after free. I have a new branch to PR once we get some more experience with it.

[zwoop]

Please remember to remove Milestone and Projects from PRs that are closed without merging.