Seg Fault on JPEG2000 decoding

Simple Description on the Bug

There is a segmentation fault when reading JPEG2000 streams due to unsafe code. This is due to the image struct being dropped at the end of the function, so the pointer given to from_raw_parts is no longer valid.

https://github.com/noritada/grib-rs/blob/1b3d559bf6215acf656063fa51bbdb4c5b83ee4b/src/decoders/jpeg2000/mod.rs#L113-L119

I found this when trying to read data from the ECMWF provided files (namely the U/V components of wind)

Steps to Reproduce

Download some of the U/V component files from ECMWF
Run gribber decode <file.grib2.bin> 0

Expected Behavior

I get a nice array of values

Actual Behavior

Segfault on Linux

Additional Context

A trivial way would be to create a vec so that the values are owned:

        let vec = unsafe {
            std::slice::from_raw_parts(comp_gray.data, (width * height) as usize)
                .iter()
                .map(|x| *x as i32)
                .collect::<Vec<_>>()
        };
        Ok(vec.into_iter())

Or the SimplePackingDecodeIterator could be inlined into the function, ensuring the slice memory is alive until the return

noritada / grib-rs