The shared UTF-8 decoder behind JSStringCreateWithUTF8CString and
JSStringIsEqualToUTF8CString now applies StringImpl::create's bounds: more
than INT32_MAX bytes throws before decoding, and non-ASCII input decoding to
more than 2147483635 UTF-16 units throws after a successful conversion.
GuestJSStringValidLength mirrors isValidLength and is tested at both limits.
JSStringCreateWithUTF8CString and JSStringIsEqualToUTF8CString take a null
UTF-8 pointer as a null string, and JSStringCreateWithCharacters returns a
null string for null characters of any length, as WebKit's JSStringRef.cpp
and WTF's String constructors do. A length above WebKit's 2147483635-unit
limit and a null handle given to retain, release or the maximum-size query
throw std exceptions where JavaScriptCore crashes. The reference count is a
wrapping 32-bit counter and the maximum UTF-8 size is computed in 32 bits,
as in WTF.
simnJS