libcacard/vreader: Tighten assertion to clarify intent

Bonus: hushes up Coverity.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Alon Levy <alevy@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>