Fix integer underflow in RawIter::size_hint

[nwtnni]

I encountered this bug while trying to collect ThreadLocal::iter into a Vec and occasionally getting a capacity overflow error (backtrace attached below). The root cause is that RawIter yields based on Entry.present, which is updated before ThreadLocal.values; this means it's possible for self.yielded > thread_local.values.

I wasn't able to reproduce the problem in a unit test (tried under miri as well) since the conflict window is so short--would probably need loom or some other adversarial scheduler.

Vec overflow backtrace

thread '<unnamed>' panicked at /nix/store/yhsr8dw2nyhga4jy8bif080kazd3syh6-rust-default-1.88.0/lib/rustlib/src/rust/library/alloc/src/raw_vec/mod.rs:558:17:
capacity overflow
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/std/src/panicking.rs:697:5
   1: core::panicking::panic_fmt
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/core/src/panicking.rs:75:14
   2: alloc::raw_vec::capacity_overflow
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/alloc/src/raw_vec/mod.rs:29:5
   3: alloc::raw_vec::handle_error
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/alloc/src/raw_vec/mod.rs:797:29
   4: alloc::raw_vec::RawVecInner<A>::reserve::do_reserve_and_handle
             at /nix/store/yhsr8dw2nyhga4jy8bif080kazd3syh6-rust-default-1.88.0/lib/rustlib/src/rust/library/alloc/src/raw_vec/mod.rs:558:17
   5: alloc::raw_vec::RawVecInner<A>::reserve
             at /nix/store/yhsr8dw2nyhga4jy8bif080kazd3syh6-rust-default-1.88.0/lib/rustlib/src/rust/library/alloc/src/raw_vec/mod.rs:563:13
   6: alloc::raw_vec::RawVec<T,A>::reserve
             at /nix/store/yhsr8dw2nyhga4jy8bif080kazd3syh6-rust-default-1.88.0/lib/rustlib/src/rust/library/alloc/src/raw_vec/mod.rs:331:20
   7: alloc::vec::Vec<T,A>::reserve
             at /nix/store/yhsr8dw2nyhga4jy8bif080kazd3syh6-rust-default-1.88.0/lib/rustlib/src/rust/library/alloc/src/vec/mod.rs:1297:18
   8: alloc::vec::Vec<T,A>::extend_desugared
             at /nix/store/yhsr8dw2nyhga4jy8bif080kazd3syh6-rust-default-1.88.0/lib/rustlib/src/rust/library/alloc/src/vec/mod.rs:3550:17
   9: <alloc::vec::Vec<T,A> as alloc::vec::spec_extend::SpecExtend<T,I>>::spec_extend
             at /nix/store/yhsr8dw2nyhga4jy8bif080kazd3syh6-rust-default-1.88.0/lib/rustlib/src/rust/library/alloc/src/vec/spec_extend.rs:19:9
  10: <alloc::vec::Vec<T> as alloc::vec::spec_from_iter_nested::SpecFromIterNested<T,I>>::from_iter
             at /nix/store/yhsr8dw2nyhga4jy8bif080kazd3syh6-rust-default-1.88.0/lib/rustlib/src/rust/library/alloc/src/vec/spec_from_iter_nested.rs:42:9
  11: <alloc::vec::Vec<T> as alloc::vec::spec_from_iter::SpecFromIter<T,I>>::from_iter
             at /nix/store/yhsr8dw2nyhga4jy8bif080kazd3syh6-rust-default-1.88.0/lib/rustlib/src/rust/library/alloc/src/vec/spec_from_iter.rs:34:9
  12: <alloc::vec::Vec<T> as core::iter::traits::collect::FromIterator<T>>::from_iter
             at /nix/store/yhsr8dw2nyhga4jy8bif080kazd3syh6-rust-default-1.88.0/lib/rustlib/src/rust/library/alloc/src/vec/mod.rs:3438:9
  13: core::iter::traits::iterator::Iterator::collect
             at /nix/store/yhsr8dw2nyhga4jy8bif080kazd3syh6-rust-default-1.88.0/lib/rustlib/src/rust/library/core/src/iter/traits/iterator.rs:2001:9