Fix UB in IOChunk::as_str(): only cache utf8_verified on successful UTF-8 validation

[Copilot]

IOChunk::as_str() set utf8_verified = true unconditionally — even when str::from_utf8 failed. A subsequent call would then hit the cached branch and invoke as_str_unchecked() on invalid UTF-8, which is undefined behavior.

Change

• src/sed/fast_io.rs: Replace the unconditional utf8_verified.set(true) with a match on the str::from_utf8 result, only setting the flag in the Ok arm.

// Before
let result = str::from_utf8(content);
self.utf8_verified.set(true);          // ← set even on Err
result.map_err(|e| USimpleError::new(2, e.to_string()))

// After
match str::from_utf8(content) {
    Ok(s) => {
        self.utf8_verified.set(true);  // ← only on success
        Ok(s)
    }
    Err(e) => Err(USimpleError::new(2, e.to_string())),
}

[dspinellis]

Incorporated into #487.