Don't free up EVP_MD_CTX.

Don't free up passed EVP_MD_CTX in ASN1_item_sign_ctx(). This
simplifies handling and retains compatiblity with previous behaviour.

PR#4446

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Stephen Henson <steve@openssl.org>