Access log: fix default value, broken by cb308813b453.

log->filter ("if" parameter) was uninitialized when the default value
was being used, which would lead to a crash (SIGSEGV) when access_log
directive wasn't specified in the configuration.

Zero-fill the whole structure instead of zeroing fields one-by-one
in order to prevent similar issues in the future.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>