SL-10285 - removed one possible route for the isImpostor() crash. Intermittent issue.

indra/newview/llcontrolavatar.cpp

@@ -60,6 +60,8 @@ LLControlAvatar::LLControlAvatar(const LLUUID& id, const LLPCode pcode, LLViewer
 // virtual
 LLControlAvatar::~LLControlAvatar()
 {
+	// Should already have been unlinked before destruction
+	llassert(!mRootVolp);
 }
 
 // virtual

indra/newview/llviewerobject.cpp

@@ -3182,6 +3182,7 @@ void LLViewerObject::unlinkControlAvatar()
         if (mControlAvatar)
         {
             mControlAvatar->markForDeath();
+			mControlAvatar->mRootVolp = NULL;
             mControlAvatar = NULL;
         }
     }