add a check for the return of sk_SRP_gN_new_null() so that capture the...

add a check for the return of sk_SRP_gN_new_null() so that capture the potential memory error in time

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19435)