keep error checking style with setgroups logic

The code here all checks != 0, so make the new code do the same.

Bug: None
Test: make check still passes
Change-Id: I94c6032431eea8bc0b017b8d3e1d4f6a6e98506e
diff --git a/libminijail.c b/libminijail.c
index 26787e3..3b0c3e1 100644
--- a/libminijail.c
+++ b/libminijail.c
@@ -1298,7 +1298,7 @@
 	if (j->gidmap && j->flags.disable_setgroups) {
 		/* Older kernels might not have the /proc/<pid>/setgroups files. */
 		int ret = write_proc_file(j->initpid, "deny", "setgroups");
-		if (ret < 0) {
+		if (ret != 0) {
 			if (ret == -ENOENT) {
 				/* See http://man7.org/linux/man-pages/man7/user_namespaces.7.html. */
 				warn("could not disable setgroups(2)");