APPS: Add missing OPENSSL_free() and combine the error handler

APPS: Add missing OPENSSL_free() and combine the error handler

Add the OPENSSL_free() in the error handler to release the "*md_value"
allocated by app_malloc(). To make the code clear and avoid possible
future errors, combine the error handler in the "err" tag.
Then, we only need to use "goto err" instead of releasing the memory
separately.

Since the EVP_MD_get_size() may return negative numbers when an error occurs,
create_query() may fail to catch the error since it only considers 0 as an
error code.

Therefore, unifying the error codes of create_digest() from non-positive
numbers to 0 is better, which also benefits future programming.

Fixes: c7235be ("RFC 3161 compliant time stamp request creation, response generation and response verification.")
Signed-off-by: Jiasheng Jiang <jiasheng@purdue.edu>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/23873)