Fix a panic in Mp4Track::read_sample() for one-past-the-end

This appears to be a bug unmasked by other changes. read_sample() calls
sample_offset() then sample_size(), and assumes that if the former returns Ok
then the latter does as well. However, if the sample_id is one past the end,
sample_offset() might succeed (it only checks samples _up to_ the given
sample_id but not _including_ it) while sample_size() fails (because the sample
doesn't exist). read_sample() will then panic.

Fix this by duplicating the error propagation (that is currently done for
sample_offset) for sample_size, instead of unwrapping. This is a cautious
change that fixes the bug; alternatively, having sample_offset() call
sample_size() on the given sample_id and propagate any error might also work.

src/track.rs

@@ -540,7 +540,11 @@ impl Mp4Track {
             Err(Error::EntryInStblNotFound(_, _, _)) => return Ok(None),
             Err(err) => return Err(err),
         };
-        let sample_size = self.sample_size(sample_id).unwrap();
+        let sample_size = match self.sample_size(sample_id) {
+            Ok(size) => size,
+            Err(Error::EntryInStblNotFound(_, _, _)) => return Ok(None),
+            Err(err) => return Err(err),
+        };
 
         let mut buffer = vec![0x0u8; sample_size as usize];
         reader.seek(SeekFrom::Start(sample_offset))?;