fix panic when requesting zero segment duration

The recording::Segment was constructing a segment with no frames in it, which
was causing a panic when appending a zero-length stts to the Slices. Fix this
in a couple ways:

* Slices::append should return Err rather than panic. No reason to crash the
  whole program when we have trouble serving a single .mp4 request.
* recording::Segment shouldn't produce zero-frame segments

src/mp4.rs

@@ -1405,8 +1405,7 @@ impl BodyState {
 
     fn append_slice(&mut self, len: u64, t: SliceType, p: usize) -> Result<(), Error> {
         let l = self.slices.len();
-        self.slices.append(Slice::new(l + len, t, p)?);
-        Ok(())
+        self.slices.append(Slice::new(l + len, t, p)?)
     }
 
     /// Appends a static bytestring, flushing the buffer if necessary.