Fixed normal bug EXT-6624 ('Land for sale' icon isn't showed in Navigation bar...

Fixed normal bug EXT-6624 ('Land for sale' icon isn't showed in Navigation bar after teleport from another land for sale with 'Moderate' rating)

-       Reason: after teleport, Agent's content rate compares with just teleported Region's content rate, to check whether Agent can buy a Parcel in this Region. But it compares with old Region ones. Because In method 'LLViewerParcelMgr::canAgentBuyParcel' the call of 'LLViewerParcelMgr::getInstance()->getSelectionRegion()' returns last selected parcel. So after teleport this method call will always return old Region.
        Solution: In 'LLViewerParcelMgr::canAgentBuyParcel' determine current Parcel Agent is in, then determine Region using Agent's Parcel information and use actual Region.

Reviewed by Vadim Savchuk at https://codereview.productengine.com/secondlife/r/222/

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branch : product-engine