fix computing duration for each task

src/main.rs

@@ -91,11 +91,12 @@ fn log(f: &String, enabled: &bool, since: &Option<String>) -> Result<(), String>
          curr.t = x.timestamp();
          curr.d = 0.0;
       }
-      let d = match curr.xs.len() {
-         0 if x.x.len() == 0 => 0.0,
-         0 => 1.0,
-         _ => ((tsheet.xs[i].t - tsheet.xs[i-1].t) as f32 / (60.0*60.0)) as f32,
-      };
+      let mut d = 1.0;
+      if x.x.len() == 0 {
+         d = 0.0;
+      } else if i > 0 {
+         d = ((tsheet.xs[i].t - tsheet.xs[i-1].t) as f32 / (60.0*60.0)) as f32;
+      }
       match x.x.len() {
          0 => {},
          _ => {