`<deque>`: Add missed masking of `_First_used_block_idx` in `deque::shrink_to_fit` to correctly end the loop#4955

StephanTLavavej merged 2 commits intomicrosoft:mainmicrosoft/STL:mainfrom

frederick-vs-ja:deque-missed-maskfrederick-vs-ja/STL:deque-missed-maskCopy head branch name to clipboard