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)

indra/newview/llfloateravatarpicker.cpp

@@ -387,8 +387,8 @@ void LLFloaterAvatarPicker::processAvatarPickerReply(LLMessageSystem* msg, void*
 	
 	LLFloaterAvatarPicker* floater = LLFloaterReg::findTypedInstance<LLFloaterAvatarPicker>("avatar_picker");
 
-	// these are not results from our last request
-	if (query_id != floater->mQueryID)
+	// floater is closed or these are not results from our last request
+	if (NULL == floater || query_id != floater->mQueryID)
 	{
 		return;
 	}