selftests/bpf: Fix warning comparing pointer to 0

Fix the following coccicheck warning:

./tools/testing/selftests/bpf/progs/test_global_func10.c:17:12-13:
WARNING comparing pointer to 0.

Reported-by: Abaci Robot <[e-mail]>
Signed-off-by: Jiapeng Chong <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/progs/test_global_func10.c

···
 
 __noinline int foo(const struct Big *big)
 {
-	if (big == 0)
+	if (!big)
 		return 0;
 
 	return bpf_get_prandom_u32() < big->y;