core.app: fix bug in reconfiguration

If an app network is re-configured, the 'keep' action copies the
existing input and output tables, but the code that processes the
links assumes that these tables are empty in the new app table.  This
causes the array-part of the tables to be duplicated.

src/core/app.lua

@@ -170,6 +170,9 @@ function apply_config_actions (actions, conf)
    end
    function ops.keep (name)
       new_app_table[name] = app_table[name]
+      -- Clear input/output tables, they will be re-populated below
+      new_app_table[name].output = {}
+      new_app_table[name].input = {}
    end
    function ops.start (name)
       local class = conf.apps[name].class