support: Report NULL blobs explicitly in TEST_COMPARE

  * Florian Weimer:

> Provide an explicit diagnostic if the length is positive, and
> do not just crash with a null pointer dereference.  Null pointers
> are only valid if the length is zero, so this can only happen with
> a faulty test.
>
> 2019-05-03  Florian Weimer  <fweimer@redhat.com>
>
> 	* support/support_test_compare_blob.c (report_blob): Report
> 	incorrect NULL blobs.
>
> diff --git a/support/support_test_compare_blob.c b/support/support_test_compare_blob.c
> index 5bcb03418c..00491b0df1 100644
> --- a/support/support_test_compare_blob.c
> +++ b/support/support_test_compare_blob.c
> @@ -33,7 +33,9 @@ static void
>  report_blob (const char *what, const unsigned char *blob,
>               unsigned long int length, const char *expr)
>  {
> -  if (length > 0)
> +  if (blob == NULL)
> +    printf ("  %s (evaluated from %s): NULL\n", what, expr);
> +  else if (length > 0)
>      {
>        printf ("  %s (evaluated from %s):\n", what, expr);
>        char *quoted = support_quote_blob (blob, length);

Hmph, this patch is better because it does not change test failure
output with for NULL with zero length:
  

* Florian Weimer:

> * Florian Weimer:
>
>> Provide an explicit diagnostic if the length is positive, and
>> do not just crash with a null pointer dereference.  Null pointers
>> are only valid if the length is zero, so this can only happen with
>> a faulty test.
>>
>> 2019-05-03  Florian Weimer  <fweimer@redhat.com>
>>
>> 	* support/support_test_compare_blob.c (report_blob): Report
>> 	incorrect NULL blobs.

> Hmph, this patch is better because it does not change test failure
> output with for NULL with zero length:

I've pushed the second patch.

Thanks,
Florian