fixed R12 incompatibility in gen_server2

Author: Marek Majkowski

src/gen_server2.erl

@@ -437,7 +437,10 @@ unregister_name({local,Name}) ->
 unregister_name({global,Name}) ->
     _ = global:unregister_name(Name);
 unregister_name(Pid) when is_pid(Pid) ->
-    Pid.
+    Pid;
+% Under R12 let's just ignore it, as we have a single term as Name.
+% On R13 it will never get here, as we get tuple with 'local/global' atom.
+unregister_name(Name) -> ok.
 
 extend_backoff(undefined) ->
     undefined;