In reinit(), exit early if something stopped the stream

Reinit is inherently racy. It can be triggered async by the input
callback, and the output callback can, between the input callback and
reinit(), set the `stopped` flag. In this case, there is no need to
continue with the reinit.

src/backend/mod.rs

@@ -4700,6 +4700,11 @@ impl<'ctx> AudioUnitStream<'ctx> {
             self.core_stream_data.stop_audiounits();
         }
 
+        if self.stopped.load(Ordering::SeqCst) {
+            // Something stopped the stream, we must not reinit.
+            return Ok(());
+        }
+
         debug_assert!(
             !self.core_stream_data.input_unit.is_null()
                 || !self.core_stream_data.output_unit.is_null()