Fix `cert_create` tool for Segmentation fault

Fix `cert_create` tool for Segmentation fault

With the introduction of commit `96103d5a`, the Certificate
Generation tool is not able to generate FWU certificate and
while doing so it does segmentation fault.

This happens because it is now required to pass non-volatile
counter values to the `cert_create` tool from the command line
for creating the trusted firmware certificates.

But in case of creating FWU certificate these counter values are not
being passed to the tool and as a consequence the `cert_create` tool
try to use the NULL argument and errors out with Segmentation fault.

This patch fixes this issue by providing a check before using the
command line argument passed in the case of `EXT_TYPE_NVCOUNTER`
certificate extension.

Change-Id: Ie17d0c1502b52aaa8500f3659c2da2448ab0347a