Further fix in bio_dgram_test for BIO_s_dgram_mem()

Further fix in bio_dgram_test for BIO_s_dgram_mem()

When setting an explicit buffer size using BIO_s_dgram_mem() make sure we
take into account the size of the header (which may be large on NonStop)

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22058)