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 <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1615357366-97612-1-git-send-email-jiapeng.chong@linux.alibaba.com

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

@@ -14,7 +14,7 @@ struct Big {
 
 __noinline int foo(const struct Big *big)
 {
-	if (big == 0)
+	if (!big)
 		return 0;
 
 	return bpf_get_prandom_u32() < big->y;