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.
Merge pull request #3395 from garbear/invalid-dates

Fix infinite loop while viewing "EPG: Timeline" window