[gnc-tree-container.hpp] bugfix in operator==

when comparing two GncTreeIter, if they both were end() iterators from
differing GtkTreeModels they would have been deemed equal. Amend so
that operator== returns false.

gnucash/gnome-utils/gnc-tree-container.hpp

@@ -102,6 +102,8 @@ public:
 
     bool operator==(const GncTreeIter& other) const
     {
+        if (m_model != other.m_model)
+            return false;
         if (!m_iter.has_value() && !other.m_iter.has_value())
             return true;
         if (!m_iter.has_value() || !other.m_iter.has_value())