Should check for NULL before calling the callback

Node will define a callback with a large class_id, then the callback
list is filled with NULLs and later leads to crash when generating
snapshot from Webkit (class id = 2).

Fix rogerwang/node-webkit#204.

upstream issue: http://code.google.com/p/v8/issues/detail?id=2430
Merge branch 'nwebkit' of https://github.com/rogerwang/v8 into nwebkit
revert to 014ba1a (0.3.5)
merge to latest upstream 4cd70b470