Fix overflowing length in Vec<ZST> to VecDeque

`Vec` can hold up to `usize::MAX` ZST items, but `VecDeque` has a lower
limit to keep its raw capacity as a power of two, so we should check
that in `From<Vec<T>> for VecDeque<T>`. We can also simplify the
capacity check for the remaining non-ZST case.

Before this fix, the new test would fail on the length:

```
thread 'collections::vec_deque::tests::test_from_vec_zst_overflow' panicked at 'assertion failed: `(left == right)`
  left: `0`,
 right: `9223372036854775808`', library/alloc/src/collections/vec_deque/tests.rs:474:5
note: panic did not contain expected string
      panic message: `"assertion failed: `(left == right)`\n  left: `0`,\n right: `9223372036854775808`"`,
 expected substring: `"capacity overflow"`
```

That was a result of `len()` using a mask `& (size - 1)` with the
improper length. Now we do get a "capacity overflow" panic as soon as
that `VecDeque::from(vec)` is attempted.

0 files changed, 0 insertions, 0 deletions