Fix underflow when input is shorter than one sector (#141)

This also changes the error to InvalidSignature when the input is too
short (it used to be ReadError in release mode and panic in debug mode).

Author: frankdavid

src/lib.rs

@@ -729,6 +729,10 @@ impl GPT {
         reader.seek(SeekFrom::Start(sector_size))?;
         let header = GPTHeader::read_from(&mut reader).or_else(|primary_err| {
             let len = reader.seek(SeekFrom::End(0))?;
+            if len < sector_size {
+                return Err(InvalidSignature);
+            }
+
             reader.seek(SeekFrom::Start((len / sector_size - 1) * sector_size))?;
 
             GPTHeader::read_from(&mut reader).map_err(|backup_err| {
@@ -1427,6 +1431,15 @@ mod test {
         assert!(GPT::find_from(&mut fs::File::open(DISK2).unwrap()).is_ok());
     }
 
+    #[test]
+    fn input_too_short() {
+        let mut empty = io::Cursor::new(vec![1; 5]);
+        assert!(matches!(
+            GPT::read_from(&mut empty, 512).expect_err("Should fail on short input"),
+            Error::InvalidSignature
+        ));
+    }
+
     #[test]
     fn find_backup() {
         fn test(path: &str, ss: u64) {