tests: Use volatile pointer for NULL dereference

Clang warns that it can silently discard a non-volatile write to a NULL
pointer (perhaps it constitutes undefined behaviour?), and recommends
changing it to volatile.

This patch slavishly complies with the demand of the unfeeling machine.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>