When waiting for the decoder to provide more data, the player thread
generates silence chunks if needed.  However, it forgot to initialize
the chunk.times attribute, which had now an undefined value.  This
patch sets it to -1.0, meaning "value is undefined".  Add a ">= 0.0"
check to audio_output_all_check().  This fixes spurious relative
seeking errors, because sometimes, the "elapsed" value falls back to
0.0.