Minor bug fix for house system combo box

In case of a (theoretically impossible) negative value, the ID field
would have crashed the application

src/ag-window.c

@@ -1654,6 +1654,8 @@ ag_window_init(AgWindow *window)
             NULL
         );
 
+    // Fill the house system model and set the combo box on the Edit tab to the
+    // default one
     house_system_list = gswe_all_house_systems();
     g_list_foreach(house_system_list, (GFunc)ag_window_add_house_system, priv);
     g_list_free(house_system_list);

src/resources/ui/ag-window.ui

@@ -717,7 +717,7 @@
                     <property name="visible">True</property>
                     <property name="can_focus">False</property>
                     <property name="model">house_system_model</property>
-                    <property name="id_column">0</property>
+                    <property name="id_column">1</property>
                   </object>
                   <packing>
                     <property name="left_attach">5</property>