'coroutine.running' should return nil (instead of nothing) for main thread

lbaselib.c

@@ -1,5 +1,5 @@
 /*
-** $Id: lbaselib.c,v 1.190 2006/05/31 16:50:40 roberto Exp roberto $
+** $Id: lbaselib.c,v 1.191 2006/06/02 15:34:00 roberto Exp roberto $
 ** Basic library
 ** See Copyright Notice in lua.h
 */
@@ -586,8 +586,7 @@ static int luaB_costatus (lua_State *L) {
 
 static int luaB_corunning (lua_State *L) {
   if (lua_pushthread(L))
-    return 0;  /* main thread is not a coroutine */
-  else
+    lua_pushnil(L);  /* main thread is not a coroutine */
   return 1;
 }