Explicitly set log object to r->connection->log in preaccess phase handler.

This change fixes a number of scenarios when ModSecurity's log entries
may be written to the wrong file descriptors. In particular, there was
an issue with almost any configuration using nginx cache features
(proxy_cache, fastcgi_cache, etc) when garbage from ModSecurity logs
has been sent to the control socket used for communication between
nginx master process and auxiliary processes (workers, cache manager,
cache loader).

Described behavior was observed with nginx/1.7.0, modsecurity/2.8.0
and OWASP CRS v2.2.9.