v8: work around String::WriteAscii segfault

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

This commit reapplies 9668df8. The issue has been fixed upstream but
reappeared after last night's downgrade to V8 3.14.5 in commit b15a10e.

Conflicts:
	test/simple/test-buffer.js

deps/v8/src/v8utils.h

@@ -209,6 +209,8 @@ INLINE(void CopyChars(sinkchar* dest, const sourcechar* src, int chars));
 
 template <typename sourcechar, typename sinkchar>
 void CopyChars(sinkchar* dest, const sourcechar* src, int chars) {
+  ASSERT(chars >= 0);
+  if (chars == 0) return;
   sinkchar* limit = dest + chars;
 #ifdef V8_HOST_CAN_READ_UNALIGNED
   if (sizeof(*dest) == sizeof(*src)) {