[Deque]: Fix bogus assert in Deque._Storage._ensureUnique

[lorentey]

A non-unique reference to a storage instance can legitimately become unique at any point, due to other references (say, held, by another thread) getting destroyed.

Deque’s ensureUnique implementation currently invokes the uniqueness check two times in quick succession, expecting it to return false both times. In rare circumstances, this can lead to a spurious trap in concurrent environments.

Fix this by avoiding calling isUnique more than once.

(We do not have a great way to exercise such concurrency issues, so this fix comes with no test case.)

Resolves #380

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[lorentey]

@swift-ci test