fix: handle Suffix(0) as unsatisfiable instead of panicking

`Range: bytes=-0` would pass the `checked_sub` guard (since
`size - 0 = size`), then compute `start = size, end = size - 1`,
causing `Bound::new` to fail and the `unwrap()` to panic.

Validate that the computed start position is within bounds by using
`is_some_and(|start| start < size)`.

Author: paolobarbolini

src/lib.rs

@@ -78,7 +78,7 @@ pub fn file_range(
                 range: start..=end,
             })
         }
-        HttpRange::Suffix(suffix) if size.checked_sub(suffix).is_some() => {
+        HttpRange::Suffix(suffix) if size.checked_sub(suffix).is_some_and(|start| start < size) => {
             let start = size - suffix;
             let end = size - 1;
             let content_range =