demux: subtitle: check overflow when converting hour+minute+second+ms timestamps

A helper is added to factorize the conversions and the checks.

Fixes #29261 because the conversion is done an an int64_t instead of a long, but the value is still bogus.

We may add extra sanity checks on the minutes, seconds, milliseconds and use unsigned in some places. But the restrictions may not apply to all formats.

Some missing VLC_TICK_0 are added to i_stop values and are set to VLC_TICK_INVALID when not set.