Fix infinite loop while viewing "EPG: Timeline" window

With invalid begin/end times, the for loop in [GUIEPGContainer.cpp L#825](https://github.com/xbmc/xbmc/blob/5f33d290e/xbmc/epg/GUIEPGGridContainer.cpp#L825) loops endlessly, freezing XBMC when the user views the "EPG: Timeline" window. Solve this by adding check for invalid end times.

Tested with VDR backend (backend reports epoch for both begin and end times).