BUG/MEDIUM: hlua: properly handle sample func errors in hlua_run_sample_{fetch,conv}()

BUG/MEDIUM: hlua: properly handle sample func errors in hlua_run_sample_{fetch,conv}()

To execute sample fetches and converters from lua. hlua API leverages the
sample API. Prior to executing the sample func, the arg checker is called
from hlua_run_sample_{fetch,conv}() to detect potential errors.

However, hlua_run_sample_{fetch,conv}() both pass NULL as <err> argument,
but it is wrong for two reasons. First we miss an opportunity to report
precise error messages to help the user know what went wrong during the
check.. and more importantly, some val check functions consider that the
<err> pointer is never NULL. This is the case for example with
check_crypto_hmac(). Because of this, when such val check functions
encounter an error, they will crash the process because they will try
to de-reference NULL.

This bug was discovered and reported by GH user @JB0925 on #2745.

Perhaps val check functions should make sure that the provided <err>
pointer is != NULL prior to de-referencing it. But since there are
multiple occurences found in the code and the API isn't clear about that,
it is easier to fix the hlua part (caller) for now.

To fix the issue, let's always provide a valid <err> pointer when
leveraging val_arg() check function pointer, and make use of it in case
or error to report relevant message to the user before freeing it.

It should be backported to all stable versions.