Fixed bug EXT-6798 (Crash in LLTeleportHistoryStorage).

Reason: Attempt to purge empty teleport history resulted in inconsistent history state.
There were two consequences:
1) Further teleports would not properly update the history.
2) Subscribers of history changes were notified of the invalid change and that led to the crash.

My changes:
- added a sanity check to LLTeleportHistoryStorage::onTeleportHistoryChange() to prevent the crash
- purging empty teleport history now does nothing, hence the history doesn't become inconsistent
- removed a redundant (but harmless) call to onTeleportHistoryChanged() from LLNavigationBar::draw() because it's called by LLTeleportHistory::purgeItems() anyway

Reviewed by Mike at https://codereview.productengine.com/secondlife/r/304/

--HG--
branch : product-engine