Fix a use-after-free in the child provider code

Fix a use-after-free in the child provider code

If the child provider context data gets cleaned up before all usage of
providers has finished then a use-after-free can occur. We change the
priority of this data so that it gets freed later.

Fixes #15284

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15286)