Upstream: read handler cleared on upstream finalization.

Upstream: read handler cleared on upstream finalization.

With "proxy_ignore_client_abort off" (the default), upstream module changes
r->read_event_handler to ngx_http_upstream_rd_check_broken_connection().
If the handler is not cleared during upstream finalization, it can be
triggered later, causing unexpected effects, if, for example, a request
was redirected to a different location using error_page or X-Accel-Redirect.
In particular, it makes "proxy_ignore_client_abort on" non-working after
a redirection in a configuration like this:

    location = / {
        error_page 502 = /error;
        proxy_pass http://127.0.0.1:8082;
    }

    location /error {
        proxy_pass http://127.0.0.1:8083;
        proxy_ignore_client_abort on;
    }

It is also known to cause segmentation faults with aio used, see
http://mailman.nginx.org/pipermail/nginx-ru/2015-August/056570.html.

Fix is to explicitly set r->read_event_handler to ngx_http_block_reading()
during upstream finalization, similar to how it is done in the request body
reading code and in the limit_req module.