DecoderBuffer: add method _available()

src/DecoderBuffer.hxx

@@ -77,6 +77,13 @@ decoder_buffer_clear(DecoderBuffer *buffer);
 bool
 decoder_buffer_fill(DecoderBuffer *buffer);
 
+/**
+ * How many bytes are stored in the buffer?
+ */
+gcc_pure
+size_t
+decoder_buffer_available(const DecoderBuffer *buffer);
+
 /**
  * Reads data from the buffer.  This data is not yet consumed, you
  * have to call decoder_buffer_consume() to do that.  The returned