refactor: use error for overflow

Reworks the overflow handling to return an error instead of using the
saturating arithmetic strategy.

src/patch.rs

@@ -49,7 +49,9 @@ pub fn patch<T: Read>(old: &[u8], patch: &mut T, new: &mut Vec<u8>) -> io::Resul
         let seek_len = offtin(buf[16..24].try_into().unwrap());
 
         // Read diff string and literal data at once
-        let to_read = copy_len.saturating_add(mix_len);
+        let to_read = copy_len
+            .checked_add(mix_len)
+            .ok_or(io::Error::from(io::ErrorKind::InvalidData))?;
         let mix_start = new.len();
         let has_read = patch.take(to_read as u64).read_to_end(new)?;
 
@@ -58,15 +60,31 @@ pub fn patch<T: Read>(old: &[u8], patch: &mut T, new: &mut Vec<u8>) -> io::Resul
             return Err(io::ErrorKind::UnexpectedEof.into());
         }
 
-        let mix_slice = new.get_mut(mix_start..mix_start.saturating_add(mix_len)).ok_or(io::ErrorKind::UnexpectedEof)?;
-        let old_slice = old.get(oldpos..oldpos.saturating_add(mix_len)).ok_or(io::ErrorKind::UnexpectedEof)?;
+        let mix_slice = new
+            .get_mut(
+                mix_start
+                    ..mix_start
+                        .checked_add(mix_len)
+                        .ok_or(io::Error::from(io::ErrorKind::InvalidData))?,
+            )
+            .ok_or(io::ErrorKind::UnexpectedEof)?;
+        let old_slice = old
+            .get(
+                oldpos
+                    ..oldpos
+                        .checked_add(mix_len)
+                        .ok_or(io::Error::from(io::ErrorKind::InvalidData))?,
+            )
+            .ok_or(io::ErrorKind::UnexpectedEof)?;
         for (n, o) in mix_slice.iter_mut().zip(old_slice) {
             *n = n.wrapping_add(*o);
         }
 
         // Adjust pointers
         oldpos += mix_len;
-        oldpos = (oldpos as i64).saturating_add(seek_len) as usize;
+        oldpos = (oldpos as i64)
+            .checked_add(seek_len)
+            .ok_or(io::Error::from(io::ErrorKind::InvalidData))? as usize;
     }
 }