weston-launch: On error putenv returns a non-zero value

According to POSIX standard "upon successful completion, putenv() shall
return 0; otherwise, it shall return a non-zero value." Unlike in
setenv() we should not be checking only for negative values.

Signed-off-by: Dawid Gajownik <gajownik@gmail.com>

src/weston-launch.c

@@ -603,7 +603,7 @@ setup_session(struct weston_launch *wl)
 	env = pam_getenvlist(wl->ph);
 	if (env) {
 		for (i = 0; env[i]; ++i) {
-			if (putenv(env[i]) < 0)
+			if (putenv(env[i]) != 0)
 				error(0, 0, "putenv %s failed", env[i]);
 		}
 		free(env);