Prevent subtract with overflow

This changes assumes this only happens when the archive is invalid.

Fixes #40

src/read.rs

@@ -85,7 +85,9 @@ impl<R: Read+io::Seek> ZipArchive<R>
 
         // Some zip files have data prepended to them, resulting in the offsets all being too small. Get the amount of
         // error by comparing the actual file position we found the CDE at with the offset recorded in the CDE.
-        let archive_offset = cde_start_pos - footer.central_directory_size - footer.central_directory_offset;
+        let archive_offset = cde_start_pos.checked_sub(footer.central_directory_size)
+            .and_then(|x| x.checked_sub(footer.central_directory_offset))
+            .ok_or(ZipError::InvalidArchive("Invalid central directory size or offset"))?;
 
         let directory_start = (footer.central_directory_offset + archive_offset) as u64;
         let number_of_files = footer.number_of_files_on_this_disk as usize;