fix panic when empty string is provided and speed is set

src/duration.rs

@@ -91,6 +91,11 @@ impl DurationEstimator {
         let rho = (target_length as f64 - mean) / vari;
 
         let mut duration = Self::estimate_duration(duration_params, rho);
+        if duration.len() == 0 {
+            // If there is no duration that can be adjusted to match frame_length,
+            // simply return an empty duration
+            return vec![];
+        }
 
         // loop estimation
         let mut sum: usize = duration.iter().sum();

src/lib.rs

@@ -136,8 +136,13 @@ mod tests {
 
     #[test]
     fn empty() {
-        let engine = Engine::load(&[MODEL_NITECH_ATR503]).unwrap();
+        let mut engine = Engine::load(&[MODEL_NITECH_ATR503]).unwrap();
         let labels: [&str; 0] = [];
+
+        let speech = engine.synthesize(&labels[..]).unwrap();
+        assert_eq!(speech.len(), 0);
+
+        engine.condition.set_speed(1.2);
         let speech = engine.synthesize(&labels[..]).unwrap();
         assert_eq!(speech.len(), 0);
     }