connection_read_handler: fix potential double free

In connection_read_handler(), make sure we act accordingly when
r->connection->error is set (indicating the the current request has
been finalized).

Reproduction of what happens when we don't: enable IPRO+SPDY, and
rapidly refresh a page with chrome. These rapid abortions will
eventually trigger a segfault/hang/misc bad behaviour.
@chaizhenhua I think there are multiple paths leading up to `r->connection->error` being set with SPDY, but this the one I'm seeing most: `ngx_http_spdy_state_rst_stream()`:
https://github.com/nginx/nginx/blob/1176952193ccf47078dc84b8494d0496ad1ac4a2/src/http/ngx_http_spdy.c#L1734

Considering that, does this change LGTY?

http://www.chromium.org/spdy/spdy-protocol/spdy-protocol-draft3#TOC-2.6.3-RST_STREAM