output/alsa: add full check before calling FillWithSilence()

src/lib/alsa/PeriodBuffer.hxx

@@ -123,6 +123,8 @@ public:
 	 */
 	void FillWithSilence(const uint8_t *_silence,
 			     const size_t frame_size) noexcept {
+		assert(!IsFull());
+
 		size_t partial_frame = tail % frame_size;
 		auto *dest = GetTail() - partial_frame;
 

src/output/plugins/AlsaOutputPlugin.cxx

@@ -746,7 +746,7 @@ AlsaOutput::DrainInternal()
 	/* drain ring_buffer */
 	CopyRingToPeriodBuffer();
 
-	if (period_buffer.WasConsumed())
+	if (period_buffer.WasConsumed() && !period_buffer.IsFull())
 		/* generate some silence to finish the partial
 		   period */
 		period_buffer.FillWithSilence(silence, out_frame_size);