Fixed major bug EXT-6802 (Cancelling out of People Chooser while Searching Causes Crash)

* Added check of Avatar Picker floater (People Chooser) against NULL in search callback.

Reviewed by Vadim at https://codereview.productengine.com/secondlife/r/236/
(transplanted from 480f67e4eac0223626ce549a20760e81c4bce5b1)