check_file_size(): Fixed wrong file name on logging

we've tried to open `path' file and in case of fail, we log that
we failed to open `log_file' instead of `path'

Signed-off-by: Ricardo Maraschini <ricardo.maraschini@gmail.com>
naemon/checks.c: returning OK earlier when checking service

if run_async_service_check() returns OK, we should return
immediately, making code easier to read and avoiding
identation madness

Signed-off-by: Ricardo Maraschini <ricardo.maraschini@gmail.com>