STORE: clear err after ossl_store_get0_loader_int

STORE: clear err after ossl_store_get0_loader_int

This commit clears the error that might have been set when
ossl_store_get0_loader_int has been called as it will try to retrieve
a loader for the scheme on an empty store, which will cause the error
OSSL_STORE_R_UNREGISTERED_SCHEME to be set.

The motivation for this after returning from
ossl_store_get0_loader_int, OSSL_STORE_attach will continue and try to
fetch a OSSL_STORE_LOADER from the provider.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/12901)