Unnecessary unsafety in HDR decoder

[64]

image/src/hdr/decoder.rs

Line 346 in 984e092

ret.set_len(pixel_count);

Vec::set_len leads to the creation of uninitialized memory which can be dropped if an error occurs during decoding. Additionally the other functions like decode_component have to be very careful to overwrite every single byte in the buffer lest uninitialized memory leak to the outside.

Is there any reason why vec![0; n] can't be used here? I doubt there would be a significant performance impact, if at all.

[HeroicKatora]

It's indeed not ok, maybe worse than you'd think since the type of ret is not a Vec<u8> but a generic Vec<impl Send>. @fintelia I believe the necessary changes were part of the original #887 but dropped for the 0.21 branch since they change the interface. Now would be a good time for a rebase of those since 0.22 is delayed either way.

[HeroicKatora]

Indeed: this PR removes it https://github.com/image-rs/image/pull/887/files#diff-9e9af5d18e72ec657d6b3c02cf35c490 (marking as draft since implementation exists).