Fixed normal bug EXT-6464 (Buy pass from popup menu doesn't work).

- The bug was caused by deselection of parcel(because of zero ref count to parcel selection) after appearance of alert with ok/cancel.
Added setting pointer to this selection before notification appearance and nullifying pointer it after user chooses ok or cancel. Nullifying
will always take place because alert is modal, so there is no chance that LLPanelLandGeneral::cbBuyPass() won't be called.

Reviewed by Mike at https://codereview.productengine.com/secondlife/r/180/

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branch : product-engine