'lua_setlocal' should not pop value when failing (to be consistent

with the manual and with 'lua_setupvalue')

ldebug.c

@@ -1,5 +1,5 @@
 /*
-** $Id: ldebug.c,v 2.108 2014/12/08 15:48:23 roberto Exp roberto $
+** $Id: ldebug.c,v 2.109 2014/12/10 11:30:09 roberto Exp roberto $
 ** Debug Interface
 ** See Copyright Notice in lua.h
 */
@@ -167,9 +167,10 @@ LUA_API const char *lua_setlocal (lua_State *L, const lua_Debug *ar, int n) {
   StkId pos = 0;  /* to avoid warnings */
   const char *name = findlocal(L, ar->i_ci, n, &pos);
   lua_lock(L);
-  if (name)
+  if (name) {
     setobjs2s(L, pos, L->top - 1);
-  L->top--;  /* pop value */
+    L->top--;  /* pop value */
+  }
   lua_unlock(L);
   return name;
 }