epg timeline: Fix crash when no epg

Program terminated with signal 11, Segmentation fault.
#0  EPG::CGUIEPGGridContainer::SelectItemFromPoint (this=this@entry=0x3286970, point=..., justGrid=justGrid@entry=false) at GUIEPGGridContainer.cpp:1317
1317      if (!m_gridIndex[channel + m_channelOffset][block + m_blockOffset].item)