PVR: Fix crash in GUIEPGGridContainer

Program terminated with signal 11, Segmentation fault.
1326      if (!m_gridIndex[channelIndex][blockIndex].item)

Program terminated with signal 11, Segmentation fault.
1556      if (!item)

Refactor m_gridIndex using vector of vectors.
Merge pull request #3382 from janbar/fix_epggrid_crash

PVR: FIx crash in GUIEPGGridContainer