[epg] Fix navigating to the sidebar with cursor keys

When setting epg.lingertime to zero (which means past EPG data is disabled and you can access the sidebar by pressing left when you're at the leftmost item) two Left-presses are required to open the sidebar if you've previously moved up or down in the timeline. If you move one programme to the right on a channel and then start going left the sidebar opens on the first attempt as expected.

This is because m_item is wrongly set when changing channels (ie. moving up and down) because the m_channelCursor variable is only updated after the new item has been determined, which causes the wrong item to end up as m_item.

@xhaggi could you review?

If https://github.com/xbmc/xbmc/pull/4526 gets merged for Gotham I think this should too.