Fix numerical rollover in audio memory management

teensy3/AudioStream.cpp

 	uint32_t n, index, avail;
 	uint32_t *p, *end;
 	audio_block_t *block;
-	uint8_t used;
+	uint32_t used;
 
 	p = memory_pool_available_mask;
 	end = p + NUM_MASKS;