Fix crash with queries to proxy-node schemas

Querying the fields of a model with a proxy node, with the option of
report-all causes a crash in apteryx-xml/schema.c in the function
_sch_traverse_nodes. This is due to schema being set to NULL while
handling the proxy node. This change exits the routine early if schema
is set to NULL.

schema.c

@@ -3149,6 +3149,12 @@ _sch_traverse_nodes (sch_instance * instance, sch_node * schema, GNode * parent,
             g_free (name);
             name = sch_name (schema);
         }
+        else
+        {
+            /* This can happen if a query is for a field of the proxy schema itself */
+            rc = false;
+            goto exit;
+        }
         depth++;
     }