lavf/wavdec: Fix seeking in files with unaligned offsets

A file with a prepended ID3 tag of an uneven length was found
in the wild.

Check if the wav data starts at an uneven offset and use that
information to correct the seeking calculation in wav_seek_tag,
which used to only seek to even byte positions.

Regression since ac87eaf856e0fb51917266b899bb15d19b907baf

Signed-off-by: Alexander Strasser <eclipse7@gmx.net>
