hashbrown try_reserve test fails under miri

[gnzlbg]

It seems that miri fails when someone tries to allocate too much memory, instead of returning an error. I'll try to minimize this.

[gnzlbg]

MWE (playground):

fn main() {
    let layout = std::alloc::Layout::from_size_align(usize::max_value(), 1).unwrap();
    let ptr: *mut u8 = unsafe { std::alloc::alloc(layout) };
    assert!(ptr.is_null());
}

fails with:

memory allocation of 18446744073709551615 bytes failed

[RalfJung]

Looks like a duplicate of #613 ? If yes, please close this.

[RalfJung]

(Closing for now, we can always reopen.)