Use NULL when calling a function with pointer parameter, initializing a
pointer and returning a pointer. This will remove the following sparse
warning at different locations (see "make C=1"):
 * warning: Using plain integer as NULL pointer
Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>