fix: try_get_int/try_get_int_le do not sign-extend n-byte integers

[teddytennant]

What

Buf::try_get_int and Buf::try_get_int_le now sign-extend the value they read for nbytes < 8, matching the behavior of the panicking Buf::get_int / Buf::get_int_le.

Why

The panicking getters sign-extend via sign_extend(self.get_uint(nbytes), nbytes). Their fallible twins instead used buf_try_get_impl!, which copies the bytes into the low bytes of a zeroed [0u8; 8] and calls i64::from_{be,le}_bytes. For nbytes < 8 the high bytes stay zero, so no sign extension happens and a negative value is returned as a wrong positive number.

For example, the 3-byte big-endian value 0xffffff (which get_int(3) returns as -1) was returned by try_get_int(3) as 16777215.

try_get_int_ne delegates to these two methods and inherited the bug; it is fixed transitively. No Buf implementation overrides these methods, so all implementors were affected.

The fix makes the fallible getters mirror their panicking counterparts:

fn try_get_int(&mut self, nbytes: usize) -> Result<i64, TryGetError> {
    Ok(sign_extend(self.try_get_uint(nbytes)?, nbytes))
}
fn try_get_int_le(&mut self, nbytes: usize) -> Result<i64, TryGetError> {
    Ok(sign_extend(self.try_get_uint_le(nbytes)?, nbytes))
}

Edge cases are preserved: nbytes > 8 still panics via try_get_uint's documented check, nbytes == 8 is an identity shift, and insufficient data still returns the same Err(TryGetError { requested, available }) as before (existing doctests unchanged).

Testing

• Added try_get_int_sign_extends to tests/test_buf.rs, covering big/little/native-endian sign extension, a positive value, full width, and the insufficient-data error shape. It fails on master (try_get_int(3) returns Ok(16777215)) and passes with this change.
• cargo test passes all suites and doctests, including the existing try_get_int/try_get_int_le doctests.
• cargo fmt --all --check is clean.

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