Soundness bugs in clipboard write callback

[Uzaaft]

ClipboardWrite has two reachable soundness issues:

1. Empty OSC 52 writes produce (null, 0), but contents() passes this to slice::from_raw_parts, which requires a non-null pointer.
2. Clipboard data is binary-safe, but ClipboardContent::from_raw converts it to &str with from_utf8_unchecked. Inputs such as OSC 52 containing //4= decode to invalid UTF-8.