Fix BIO dgram pair memleak in dgram_mem_init

The created pair was not freed if ring buf init failed.

Fixes: 3a857b953216 "Implement BIO_s_dgram_mem() reusing the BIO_s_dgram_pair() code"

Reviewed-by: Matt Caswell <matt@openssl.foundation>
Reviewed-by: Frederik Wedel-Heinen <fwh.openssl@gmail.com>
Reviewed-by: Nikola Pajkovsky <nikolap@openssl.org>
Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org>
MergeDate: Mon May 11 08:09:14 2026
(Merged from https://github.com/openssl/openssl/pull/31121)