Read audio data up to last track's end if possible

The audio data is now read up to the last track's end, rather than to
the NRG footer ("first chunk offset"), unless there are no DAOX tracks.
This should be strictly equivalent in normal cases.

src/raw_audio.rs

@@ -44,16 +44,19 @@ pub fn extract_nrg_raw_audio(in_fd: &mut File,
     const BUF_SIZE: usize = 1024 * 1024 * 4; // 4 MiB
 
     // Seek to the first audio byte
-    let skip_bytes = get_daox_track1_index1(metadata);
-    try!(in_fd.seek(SeekFrom::Start(skip_bytes)));
+    let first_audio_byte = get_daox_track1_index1(metadata);
+    try!(in_fd.seek(SeekFrom::Start(first_audio_byte)));
+
+    // Get the last audio byte
+    let last_audio_byte = get_last_audio_byte(metadata);
 
     // Open output file
     let audio_name = try!(make_output_file_name(img_path));
     let mut out_fd = try!(File::create(audio_name));
 
     // Read/write audio data
-    let mut cur_offset = skip_bytes;
-    while cur_offset + BUF_SIZE as u64 <= metadata.chunk_offset {
+    let mut cur_offset = first_audio_byte;
+    while cur_offset + BUF_SIZE as u64 <= last_audio_byte {
         let mut audio_buf = [0u8; BUF_SIZE];
 
         let mut nbytes = try!(in_fd.read(&mut audio_buf));
@@ -69,7 +72,7 @@ pub fn extract_nrg_raw_audio(in_fd: &mut File,
     }
 
     // Read/write the last bytes
-    let remaining: usize = (metadata.chunk_offset - cur_offset) as usize;
+    let remaining: usize = (last_audio_byte - cur_offset) as usize;
     let mut audio_buf = vec![0u8; remaining];
     let mut nbytes = try!(in_fd.read(&mut audio_buf));
     if nbytes != remaining {
@@ -81,7 +84,7 @@ pub fn extract_nrg_raw_audio(in_fd: &mut File,
         return Err(NrgError::AudioWriteError);
     }
 
-    assert_eq!(cur_offset, metadata.chunk_offset);
+    assert_eq!(cur_offset, last_audio_byte);
     Ok(())
 }
 
@@ -100,6 +103,23 @@ fn get_daox_track1_index1(metadata: &NrgMetadata) -> u64 {
 }
 
 
+/// Returns the number of the byte past the last audio byte in the image.
+///
+/// This byte is indicated by the `track_end` of the last DAOX track in
+/// `metadata`, if at least one track is present in the DAOX chunk.
+/// If not, `metadata.chunk_offset` is returned.
+///
+/// Note that the two values should always be identical, but you never know.
+fn get_last_audio_byte(metadata: &NrgMetadata) -> u64 {
+    if let Some(daox_chunk) = metadata.daox_chunk.as_ref() {
+        if let Some(last_track) = daox_chunk.tracks.last() {
+            return last_track.track_end;
+        }
+    }
+    metadata.chunk_offset
+}
+
+
 /// Generates the output file's name from the NRG image's name.
 ///
 /// The output file's name will be `img_path`'s base name stripped for its