Check for null pointer in DxcBlob::as(_mut)_slice()

[zopsicle]

Fixes #85.

[zopsicle]

Tbh to be completely safe these functions should also check that the blob's data is suitably aligned for T and also impose a T: bytemuck::AnyBitPattern + bytemuck::NoUninit bound. It's probably better to just make them always return [u8].

[Jasper-Bekkers]

@zopsicle PRs must be signed with GPG signature before we can merge, could you do that please? :-)