property: lock the lib ctx when updating the property definition cache

property: lock the lib ctx when updating the property definition cache

Although the store being used is adequately and properly locked, the library
context is not.  Due to the mechanisms used for fetching, it is possible for
multiple stores to live within the same library context for short periods.
This fix prevents threading issues resulting from such coincidences.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14773)