[scudo] Fix String DCHECK

This resolves an issue tripping a `DCHECK`, as I was checking for the
capacity and not the size. We don't need to 0-init the Vector as it's
done already, and make sure we only 0-out the string on clear if it's
not empty.

Differential Revision: https://reviews.llvm.org/D103716

GitOrigin-RevId: 5019b0a56588fc13e6a37c49e22a812afe6c4416
1 file changed