Fixed major bug EXT-6092 [crashhunters] Crash in LLFlatListView::selectItemPair

Reason:
 A) incorrect UP arrow handling in accordion control: invisible accordion tab was selected to handle selection.
 B) invalid std::map item (in empty map) was used to make selection in Flat List

Fix: added checks against empty map before use front/back item pair before selecting first/last items.
Also updated processing of the UP key in accordion control to not select invisible accordion tab.

Reviewed by Vadim Savchuk at https://codereview.productengine.com/secondlife/r/318/

--HG--
branch : product-engine