Fix audible click at end of Wav file playing

play_sd_wav.cpp

 		memcpy((uint8_t *)header + header_offset, p, len);
 		header_offset += len;
 		buffer_offset += len;
-		buffer_length -= len;
 		data_length -= len;
 		if (data_length > 0) return false;
 		// parse the header...
 		memcpy((uint8_t *)header + header_offset, p, len);
 		header_offset += len;
 		buffer_offset += len;
-		buffer_length -= len;
 		data_length -= len;
 		if (data_length > 0) return false;
 		if (parse_format()) {
 		memcpy((uint8_t *)header + header_offset, p, len);
 		header_offset += len;
 		buffer_offset += len;
-		buffer_length -= len;
 		data_length -= len;
 		if (data_length > 0) return false;
 		//Serial.print("chunk id = ");
 		if (size < data_length) {
 			data_length -= size;
 			buffer_offset += size;
-			buffer_length -= size;
 			return false;
 		}
 		p += data_length;
 		size -= data_length;
 		buffer_offset += data_length;
-		buffer_length -= data_length;
 		data_length = 8;
 		header_offset = 0;
 		state = STATE_PARSE3;

play_sd_wav.h

 	uint32_t bytes2millis;
 	audio_block_t *block_left;
 	audio_block_t *block_right;
-	uint16_t block_offset;
+	uint16_t block_offset;		// how much data is in block_left & block_right
 	uint8_t buffer[512];		// buffer one block of data
-	uint16_t buffer_offset;
-	uint16_t buffer_length;
+	uint16_t buffer_offset;		// where we're at consuming "buffer"
+	uint16_t buffer_length;		// how much data is in "buffer" (512 until last read)
 	uint8_t header_offset;		// number of bytes in header[]
 	uint8_t state;
 	uint8_t state_play;