v8: work around String::WriteAscii segfault

See http://code.google.com/p/v8/issues/detail?id=2493 for details.

This commit reapplies 9668df8b. The issue has been fixed upstream but
reappeared after last night's downgrade to V8 3.14.5 in commit b15a10e7.

Conflicts:
	test/simple/test-buffer.js