eng_devcrypto: expand digest failure cases

Return failure when the digest_ctx is null in digest_update and
digest_final, and when md is null in digest_final.
Signed-off-by: NEneas U de Queiroz <cote2004-github@yahoo.com>
Reviewed-by: NMatthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7585)

(cherry picked from commit 4d9f99654441e36fdcb49540a1dbc9d4c70ccb68)

crypto/engine/eng_devcrypto.c

@@ -438,6 +438,9 @@ static int digest_update(EVP_MD_CTX *ctx, const void *data, size_t count)
     if (count == 0)
         return 1;
 
+    if (digest_ctx == NULL)
+        return 0;
+
     if (digest_op(digest_ctx, data, count, NULL, COP_FLAG_UPDATE) < 0) {
         SYSerr(SYS_F_IOCTL, errno);
         return 0;
@@ -451,6 +454,8 @@ static int digest_final(EVP_MD_CTX *ctx, unsigned char *md)
     struct digest_ctx *digest_ctx =
         (struct digest_ctx *)EVP_MD_CTX_md_data(ctx);
 
+    if (md == NULL || digest_ctx == NULL)
+        return 0;
     if (digest_op(digest_ctx, NULL, 0, md, COP_FLAG_FINAL) < 0) {
         SYSerr(SYS_F_IOCTL, errno);
         return 0;