mod_admin_socket: Return error on unhandled input to prevent apparent freeze

When mod_admin_socket is loaded without mod_admin_shell, attempt to use
`prosodyctl shell` will appear to freeze after any input, since no
response is returned.

plugins/mod_admin_shell.lua

@@ -198,6 +198,7 @@ module:hook("admin/repl-input", function (event)
 	if not ok then
 		event.origin.send(st.stanza("repl-result", { type = "error" }):text(err));
 	end
+	return true;
 end);
 
 -- Console commands --