input/{async,thread}: clear the CircularBuffer when it becomes empty

First step towards fixing https://github.com/MusicPlayerDaemon/MPD/issues/2186

src/input/AsyncInputStream.cxx

@@ -200,6 +200,12 @@ AsyncInputStream::Read(std::unique_lock<Mutex> &lock,
 	memcpy(ptr, r.data, nbytes);
 	buffer.Consume(nbytes);
 
+	if (buffer.empty())
+		/* when the buffer becomes empty, reset its head and
+		   tail so the next write can fill the whole buffer
+		   and not just the part after the tail */
+		buffer.Clear();
+
 	offset += (offset_type)nbytes;
 
 	if (paused && buffer.GetSize() < resume_at)

src/input/ThreadInputStream.cxx

@@ -146,6 +146,14 @@ ThreadInputStream::Read(std::unique_lock<Mutex> &lock,
 			size_t nbytes = std::min(read_size, r.size);
 			memcpy(ptr, r.data, nbytes);
 			buffer.Consume(nbytes);
+
+			if (buffer.empty())
+				/* when the buffer becomes empty,
+				   reset its head and tail so the next
+				   write can fill the whole buffer and
+				   not just the part after the tail */
+				buffer.Clear();
+
 			wake_cond.notify_all();
 			offset += nbytes;
 			return nbytes;