demux_mkv: make sure wavpacks works with older libavcodec versions

The new wavpack packet format (see previous commit) doesn't work with
older libavcodec versions, so disable the new code in this case.

The version numbers are only approximate, since the libavcodec version
wasn't bumped with the wavpack change, but it's close enough.
demux_mkv: replace awkward goto by function call

Requires reindenting a large code block to minimize random control flow.