Fix update with stripped beams (thanks to Jose M Herrero)(EJAB-1520)

src/ejabberd_update.erl

@@ -128,8 +128,10 @@ get_new_version(Module) ->
 
 get_current_version(Module) ->
     Attrs = Module:module_info(attributes),
-    {value, {vsn, CurVsn}} = lists:keysearch(vsn, 1, Attrs),
-    CurVsn.
+    case lists:keysearch(vsn, 1, Attrs) of
+        {value, {vsn, CurVsn}} -> CurVsn;
+        _ -> unknown_version
+    end.
 
 %% @spec(Dir::string(), UpdatedBeams::[atom()]) -> {Script,LowLevelScript,Check}
 build_script(Dir, UpdatedBeams) ->