Freeze the adapter to avoid invalid deref

GTK tries to get the parent item of selected elements when unselecting.
If we don't freeze, we run the risk of crashing if a selected item is
removed before being unselected.  This causes a wx warning but a) this
is suppressed in release and b) warning message is better than crash.
See next commit for fix to avoid common case of the error

common/widgets/lib_tree.cpp

@@ -354,7 +354,9 @@ void LIB_TREE::CenterLibId( const LIB_ID& aLibId )
 
 void LIB_TREE::Unselect()
 {
+    m_tree_ctrl->Freeze();
     m_tree_ctrl->UnselectAll();
+    m_tree_ctrl->Thaw();
 }