WavPack demuxer: do not rely on index when timestamp is not in indexed range.

WavPack demuxer: do not rely on index when timestamp is not in indexed range.

This fixes the situation when there are not enough entries in the index
(e.g. on initial seek there's only one index entry in the index) and index
search returns just the last known entry. That causes seeking function just to
seek there instead of trying harder to get at the requested position.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>