ao_lavc, vo_lavc: Fix crashes in case of multiple init attempts.

When initialization failed, vo_lavc may cause an irrecoverable state in
the ffmpeg-related structs. Therefore, we reject additional
initialization attempts at least until we know a better way to clean up
the mess.

ao_lavc currently cannot be initialized more than once, yet it's good to
do consistent changes there as well.

Also, clean up uninit-after-failure handling to be less spammy.