Fix counter in benchmarking program

I changed the semantics of buffer::len() to count the total number of
samples, not the number of samples per channel. That of course results
in incorrect numbers when multiplied with the channel count, and the
quantiles were about half of what was reported for the mean. This commit
fixes the benchmark output again.

Author: ruuda

examples/bench_decode.rs

@@ -62,8 +62,7 @@ fn decode_file(data: &[u8], sample_times_ns: &mut Vec<f64>) -> (f64, f64) {
                 // Update timing information.
                 let now = PreciseTime::now();
                 let duration_ns = frame_epoch.to(now).num_nanoseconds().unwrap();
-                let num_samples = block.len() as i64 * block.channels() as i64;
-                sample_times_ns.push(duration_ns as f64 / num_samples as f64);
+                sample_times_ns.push(duration_ns as f64 / block.len() as f64);
                 frame_epoch = now;
 
                 // Recycle the buffer for the next frame. There should be a