Incorrect lower bound on size hint for `<[T]>::splitn` if n==0

[steffahn]

I’m creating this issue in order to better document the bug in std that #87974 fixes once it’s merged.

fn this_should_hold_for_non_buggy_iterators(i: impl Iterator) {
    let (lower_hint, count) = dbg!(i.size_hint().0, i.count());
    assert!(lower_hint <= count)
}

fn main() {
    this_should_hold_for_non_buggy_iterators([()].splitn(0, |_| false));
    // [src/main.rs:3] i.size_hint().0 = 1
    // [src/main.rs:3] i.count() = 0
    // thread 'main' panicked at 'assertion failed: lower_hint <= count', src/main.rs:4:5
}

(playground)

@rustbot label T-libs, A-slice, A-iterators