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.
Merge pull request #797 from pagespeed/oschaaf-finalized-requests-fix

connection_read_handler: fix potential double free