timeutils: fix bad pointer cast that leads to timestamp overwriting

In `log_msg_parse_date()` the variable `now` is of type `GTimeVal`.
Its member `tv_sec` is of type `long`, and its address was cast to a `time_t`
pointer.
This led bad argument passing to the `cached_localtime()` function.

Fixes: #304

Signed-off-by: Budai Laszlo <Laszlo.Budai@balabit.com>