Remove debug_assert which was triggering unnecessary panic

When playing an mp3 on Windows in debug mode, sometimes
self.next_frame() is not empty in SampleRateConverter, leading to a
panic

See issue: #229

src/conversions/sample_rate.rs

@@ -188,8 +188,6 @@ where
         if result.is_some() {
             result
         } else {
-            debug_assert!(self.next_frame.is_empty());
-
             // draining `self.current_frame`
             if !self.current_frame.is_empty() {
                 let r = Some(self.current_frame.remove(0));