Iterating a range of skiplist may cause memory leaks

[sticnarf]

Code:

use crossbeam_skiplist::SkipSet;

fn main() {
    let set: SkipSet<_> = [1, 3, 5].iter().cloned().collect();
    assert_eq!(set.range(2..4).count(), 1);
    set.insert(3);
}

Run with address sanitizer:

$ RUSTFLAGS='-Z sanitizer=address' cargo run
=================================================================
==1934867==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 24 byte(s) in 1 object(s) allocated from:
    #0 0x55725024bf8d in malloc /rustc/llvm/src/llvm-project/compiler-rt/lib/asan/asan_malloc_linux.cpp:145:3
    #1 0x55725027936e in alloc::alloc::alloc::hcf88d30e57cf3e2a /home/yilin/.rustup/toolchains/nightly-2021-01-25-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/alloc.rs:86:14
    #2 0x557250284f25 in crossbeam_skiplist::base::Node$LT$K$C$V$GT$::alloc::he004a3fff4341a94 /home/yilin/Code/crossbeam/crossbeam-skiplist/src/base.rs:102:19
    #3 0x557250288fa5 in crossbeam_skiplist::base::SkipList$LT$K$C$V$GT$::insert_internal::h829667128f83af99 /home/yilin/Code/crossbeam/crossbeam-skiplist/src/base.rs:885:25
    #4 0x557250287fa4 in crossbeam_skiplist::base::SkipList$LT$K$C$V$GT$::get_or_insert::h5c252f8a1f57cf55 /home/yilin/Code/crossbeam/crossbeam-skiplist/src/base.rs:474:9
    #5 0x55725026dab1 in crossbeam_skiplist::set::SkipSet$LT$T$GT$::get_or_insert::h4032e246962d79f5 /home/yilin/Code/crossbeam/crossbeam-skiplist/src/set.rs:224:20
    #6 0x55725027b633 in core::iter::traits::iterator::Iterator::collect::hc2b1afd048fa866a /home/yilin/.rustup/toolchains/nightly-2021-01-25-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/iter/traits/iterator.rs:1765:9
    #7 0x55725027d811 in skiplist_demo::main::ha4dda8363f53ed87 /home/yilin/Scratch/skiplist-demo/src/main.rs:3:27
    #8 0x5572502719da in core::ops::function::FnOnce::call_once::ha570b3b78ebd326e /home/yilin/.rustup/toolchains/nightly-2021-01-25-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
    #9 0x557250270ca6 in std::rt::lang_start::_$u7b$$u7b$closure$u7d$$u7d$::h079447501dfdfceb /home/yilin/.rustup/toolchains/nightly-2021-01-25-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:66:18

SUMMARY: AddressSanitizer: 24 byte(s) leaked in 1 allocation(s).