Fixed some stuttering/glitching in the sample playback.

src/main.rs

@@ -12,6 +12,6 @@ use crate::tracker::Tracker;
 fn main() {
     let mut tracker = Tracker::new(4, 64);
     tracker.load_file("./3266CHIP.MOD");
-    tracker.play_sample(2, 8287.0);
+    tracker.play_sample(1, 8287.0);
     //tracker.play();
 }

src/sampler.rs

@@ -42,11 +42,11 @@ impl Iterator for Sample {
         if self.sample.data.len() == 0 {
             return Some(0.0);
         }
+        let value = self.sample.data[self.num_sample];
+        self.num_sample = self.num_sample.wrapping_add(1);
         if self.num_sample >= self.sample.data.len() {
             self.num_sample = 0;
         }
-        let value = self.sample.data[self.num_sample];
-        self.num_sample = self.num_sample.wrapping_add(1);
         Some((value as i8 as f32)/(i8::MAX as f32))
     }
 }