fix - not really proper fix for http://trac.kodi.tv/ticket/15501

:warning: this PR is not really meant to be merged as is but it is indeed beyond me.

The issue is that  m_Subtitle.end_display_time seems to be some insane value of NOTICE: Insane EndTime: ***4294967295***

that was observed by adding to line 194+

```
CLog::Log(LOGNOTICE, "Insane EndTime: %ld", (unsigned int) m_Subtitle.end_display_time);
```

This crude solution fixes the symtom but not the cause.

@fritsch tried to guide me on IRC to a better solution like https://github.com/uNiversaI/xbmc/commit/c24947dc99b5b3abbb025c52fa41f91c94a3216a but that doesnt even compile and I utterly lack the skills to do better.