Avoid badarg error when running get-deps before ./configure has created src/ejabberd.app (#2103)

plugins/override_opts.erl

@@ -18,6 +18,7 @@ preprocess(Config, _Dirs) ->
 		  Val -> Val
 	      end,
     Config2 = rebar_config:set_xconf(Config, top_overrides, TopOverrides),
+    try
         Config3 = case rebar_app_utils:load_app_file(Config2, _Dirs) of
 		  {ok, C, AppName, _AppData} ->
 		      lists:foldl(fun({Type, AppName2, Opts}, Conf1) when
@@ -29,4 +30,7 @@ preprocess(Config, _Dirs) ->
 		  _ ->
 		      Config2
 	end,
-    {ok, Config3, []}.
+	{ok, Config3, []}
+    catch
+        error:badarg -> {ok, Config2, []}
+    end.