tarballs_v1 crashes on <= 2 bytes with `IndexDefect`

`tarballs_v1.openStreamImpl` reads the entire stream into `data` and, when `tarballFormat == tfDetect`, immediately inspects:

- `data[0]`
- `data[1]`

There is no prior length check. Empty input, and also one-byte input, therefore causes direct indexing into an undersized string.

```nim
import std/streams
import zippy/tarballs_v1

Tarball().open(newStringStream(""))
```

Found by GPT 5.4.