Fix a race in by_store_subject

Fix a race in by_store_subject

When looking in the stack of objects in the store we need to ensure we
are holding a read lock for the store.

Issue detected via thread sanitizer after the test from the previous
commit was added.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28385)