Remove call to mem::uninitialized/zeroed inside vorbis structs

[John2143]

As of the latest rustc 1.48.0, Decoder::<R>::new() panics at runtime with the error

attempted to zero-initialize type `librespot_tremor::tremor_sys::ov_callbacks`, which is invalid

This PR adds a close_func no-op to fill out ov_callbacks and changes DecoderData to use mem::transmute. This needs a review, but it does build and run locally.

fixes spotifyd issue Spotifyd/spotifyd/issues/719

[hselasky]

Test OK using FreeBSD-12 and rust-1.48.0.

[thotypous]

Please merge. Very annoying bug. It makes life of people packaging stuff for distros really hard.

[Johannesd3]

Suggestion: Make DecoderData #[repr(C)] and define a struct

#[repr(C)]
struct DecoderDataUninit<R> where R: Read + Seek {
    vorbis: MaybeUninit<tremor_sys::OggVorbis_File>,
    reader: R,
    current_logical_bitstream: libc::c_int,
    read_error: Option<io::Error>,
}

After the vorbis field is initialized, it can be transmuted to DecoderData.

[ericonr]

Is this PR being made for the correct branch? The tagged release (https://github.com/librespot-org/librespot-tremor/tree/v0.1.0) doesn't belong to any branch in this repository.

[Johannesd3]

I created another PR (#2) based on this one, which has probably a more sound use of unsafe and also some additional improvements.

[sashahilton00]

Is this PR being made for the correct branch? The tagged release (https://github.com/librespot-org/librespot-tremor/tree/v0.1.0) doesn't belong to any branch in this repository.

This is the correct branch. I just pushed the commits. I must have somehow uploaded the tags without the commits. Thanks for spotting that.

I created another PR (#2) based on this one, which has probably a more sound use of unsafe and also some additional improvements.

Closing in favour of PR #2