Fix a race in ossl_provider_add_to_store()

Fix a race in ossl_provider_add_to_store()

If two threads both attempt to load the same provider at the same time,
they will first both check to see if the provider already exists. If it
doesn't then they will both then create new provider objects and call the
init function. However only one of the threads will be successful in adding
the provider to the store. For the "losing" thread we should still return
"success", but we should deinitialise and free the no longer required
provider object, and return the object that exists in the store.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15854)