protect ossl cleanup from multithreading errors

[baentsch]

Fixes #1471

• [no] Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• [no] Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in oqs-provider, OQS-OpenSSL, OQS-BoringSSL, and OQS-OpenSSH will also need to be ready for review and merge by the time this is merged.)

[baentsch]

@dstebila CI seems to pass, so setting this to Ready for Review as I log off finally for the day. This needs to be in the release.
@beldmit: I now see why you were wary about threading... :-/ Would be glad if you'd also take a look whether this is OK/better.

[beldmit]

LGTM. Do you have any multithreading tests?

[baentsch]

LGTM. Do you have any multithreading tests?

Excellent question. AFAIK No. Created #1474 to track/improve.