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.
@oschaaf 
1. the problem is that `r->connection->error` being set some where, and this handler is called before normal request handler, right? 
2. `ngx_http_finalize_request` is called when this handler is active, I think `r->connection->error` is already handled in `ngx_http_finalize_request/ngx_http_finalize_connection`. Is that true?