Fix mem::uninitialized depricated since 1.38

[aleksmelnikov]

Fix for #826.

[aleksmelnikov]

Errors from Travis-CI:

$ rustc --version
rustc 1.35.0 (3c235d560 2019-05-20)
...
   Compiling cranelift-simplejit v0.33.0 (/home/travis/build/CraneStation/cranelift/cranelift-simplejit)
error[E0658]: use of unstable library feature 'maybe_uninit' (see issue #53491)
  --> cranelift-simplejit/src/memory.rs:34:46
   |
34 |             let mut ptr: *mut libc::c_void = mem::MaybeUninit::uninit().assume_init();
   |                                              ^^^^^^^^^^^^^^^^^^^^^^^^
   Compiling cranelift-filetests v0.33.0 (/home/travis/build/CraneStation/cranelift/cranelift-filetests)
error[E0658]: use of unstable library feature 'maybe_uninit' (see issue #53491)
  --> cranelift-simplejit/src/memory.rs:34:73
   |
34 |             let mut ptr: *mut libc::c_void = mem::MaybeUninit::uninit().assume_init();
   |                                                                         ^^^^^^^^^^^
error: aborting due to 2 previous errors
...

[aleksmelnikov]

There's a recommendation to replace 0 as *const/mut T with ptr::null/null_mut():
rust-lang/rust#21444

[bjorn3]

Forgot about that :)

[bnjbvr]

LGTM. Can you squash your three commits together, please? Thanks!

[aleksmelnikov]

@bnjbvr , 3 commits have been squashed.

[bnjbvr]

Thanks!

[aleksmelnikov]

@bjorn3 , @bnjbvr , Thank you.