nvc0/ir: Fix assert accessing null pointer

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111007
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111167



Signed-off-by: Mark Menzynski <mmenzyns@redhat.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Tobias <Klausmann&lt;tobias.klausmann@freenet.de>
(cherry picked from commit 7493fbf0)