audio: fix encoding mode

If this code is not skipped, encoding (or dumping with --ao=pcm) will
attempt to adjust video timing to audio. Since another commit (0cce8fe6)
already avoids writing audio ahead, this didn't slow down encoding to
realtime, but it was still significantly slower.

This change should actually remove all extra sleeping.
input: fix off by one error in command parser

Should fix #989.