vc4: do not access member of a NULL structure

Check if the structure is NULL before trying to get access to its
members.

This has been detected by the Undefined Behaviour Sanitizer (UBSan).

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29911>

src/gallium/drivers/vc4/vc4_fence.c

@@ -65,7 +65,8 @@ vc4_fence_reference(struct pipe_screen *pscreen,
         struct vc4_fence *f = vc4_fence(pf);
         struct vc4_fence *old = *p;
 
-        if (pipe_reference(&(*p)->reference, &f->reference)) {
+        if (pipe_reference(old ? &old->reference : NULL,
+                           f ?  &f->reference : NULL)) {
                 if (old->fd >= 0)
                         close(old->fd);
                 free(old);