Fix UB in VecDeque::shrink_to

[kadiwa4]

See #108453

[rustbot]

r? @Mark-Simulacrum

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[thomcc]

Looks good to me. Thanks for the fix.

@bors r+ rollup

[bors]

📌 Commit 01f35b3 has been approved by thomcc

It is now in the queue for this repository.

[thomcc]

Hm, #108475 might be a better fix. This fix makes sense, but that one appears more comprehensive...

@bors r-