Fix mem leak in ECDSA_sign().

Fix mem leak in ECDSA_sign().

Similiar to the issue found in PR #20553 for DSA_sign().
ECDSA_sign() leaked memory if the signature was NULL
when i2d_ECDSA_SIG was called.

Note that this does not affect the higher level EVP
functions as they correctly handle NULL.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/20554)