use NULL instead of 0 in testcases.
Sparse, normally emit a warning when 0 is used when a pointer
is expected but here these warnings were not emitted because
errors occured before these warnings have been emitted and
sparse doesn't emit any warnings as soon as an error has been
encountered.
However, there is some changes coming regarding how previous
errors inhibit warnings and with these changes these warnings
"0 instead of NULL" will now be emitted.
Prevent to have to emit these warning by using NULL instead of 0,
as it should always be.
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>