Clear mail proxy upstream connection after close

[xintenseapple]

Summary:

• Clear the mail proxy upstream connection pointer immediately after closing it on upstream error, internal error, and session close paths.

Verification:

• git diff --check; ./auto/configure --with-mail --without-http; make -j2

Fixes #24

[mdounin]

I don't think this is needed. Handlers from the proxy module shouldn't be called after fatal errors in the first place, and this is what the patch linked in the issue comments does.