Mail: clear auth HTTP ctx before pool cleanup

[xintenseapple]

Summary:

• Route mail auth HTTP cleanup through helpers that clear the session module context before destroying the temporary pool, and guard blocked client reads against a cleared context.

Verification:

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

Fixes #23

[mdounin]

Clearing auth_http context and checking if it is set before accessing in ngx_mail_auth_http_block_read(), as the patch does, fixes the freed memory access.

The issue, however, remains: if the context is already freed and then ngx_mail_auth_http_block_read() is called, we've already queued an error message to the client (and probably placed some parts of it to the socket buffer), and calling ngx_mail_session_internal_server_error(), as in the patch, will discard the unsent part of the error message, and queue yet another error message for sending.

As suggested earlier in #23, a better fix would be to properly block reading when ngx_mail_send() is called with s->quit set.