codec: fix signature of Packet::write_interleaved

av_interleaved_write_frame never returns 1, so bool makes no sense:
https://github.com/FFmpeg/FFmpeg/blob/n4.3.1/libavformat/avformat.h#L2592-L2635

Fixes #25.

src/codec/packet/packet.rs

@@ -226,15 +226,14 @@ impl Packet {
     }
 
     #[inline]
-    pub fn write_interleaved(&self, format: &mut format::context::Output) -> Result<bool, Error> {
+    pub fn write_interleaved(&self, format: &mut format::context::Output) -> Result<(), Error> {
         unsafe {
             if self.is_empty() {
                 return Err(Error::InvalidData);
             }
 
             match av_interleaved_write_frame(format.as_mut_ptr(), self.as_ptr() as *mut _) {
-                1 => Ok(true),
-                0 => Ok(false),
+                0 => Ok(()),
                 e => Err(Error::from(e)),
             }
         }