output/alsa: replace PeriodBuffer::IsEmpty() with IsCleared()

src/lib/alsa/PeriodBuffer.hxx

@@ -65,8 +65,12 @@ public:
 		delete[] buffer;
 	}
 
-	bool IsEmpty() const noexcept {
-		return head == tail;
+	/**
+	 * Has there no data been appended since the last Clear()
+	 * call?
+	 */
+	bool IsCleared() const noexcept {
+		return tail == 0;
 	}
 
 	bool IsFull() const noexcept {

src/output/plugins/AlsaOutputPlugin.cxx

@@ -857,7 +857,7 @@ AlsaOutput::Cancel() noexcept
 		   synchronization */
 
 		pcm_export->Reset();
-		assert(period_buffer.IsEmpty());
+		assert(period_buffer.IsCleared());
 		ring_buffer->reset();
 
 		return;