PVR: FIx crash in GUIEPGGridContainer

Program terminated with signal 11, Segmentation fault.
#0  EPG::CGUIEPGGridContainer::SelectItemFromPoint (this=this@entry=0x354fd00, point=..., justGrid=justGrid@entry=false) at GUIEPGGridContainer.cpp:1326
1326      if (!m_gridIndex[channelIndex][blockIndex].item)

Don't use operator!() of item. But check attribute 'item' of pointed struct.