fix: internal pgs subtitles handling trac #15501

Fixes really insane long value for  m_Subtitle.end_display_time

Now... after comments on #5874 and fairly extensive help from @fritsch managed to get all his instructions compiled and tested.

Ive tested against http://trac.kodi.tv/ticket/15501 foreign part only pgs subs which are problematic ones.
the issue never existed with similar SRT counterparts I dont have similar ass or sub/idx to test but the issue on track is indeed resolved by this.

thank you @fritsch for actually solving it on both counts. I can say I have indeed learned something more today.