Fix in-place collect not reallocating when necessary Regression introduced in https://github.com/rust-lang/rust/pull/110353. This was [caught by miri](https://rust-lang.zulipchat.com/#narrow/stream/269128-miri/topic/Cron.20Job.20Failure.20.28miri-test-libstd.2C.202023-11.29/near/404764617) r? `@saethlin`