core/main: assign mac_init() retval to r

If it fails, we'd submit the errno to supervisor via sd_notify() later.

src/core/main.c

@@ -3136,7 +3136,8 @@ int main(int argc, char *argv[]) {
                                         goto finish;
                         }
 
-                        if (mac_init() < 0) {
+                        r = mac_init();
+                        if (r < 0) {
                                 error_message = "Failed to initialize MAC support";
                                 goto finish;
                         }
@@ -3212,7 +3213,8 @@ int main(int argc, char *argv[]) {
                 /* clear the kernel timestamp, because we are not PID 1 */
                 kernel_timestamp = DUAL_TIMESTAMP_NULL;
 
-                if (mac_init() < 0) {
+                r = mac_init();
+                if (r < 0) {
                         error_message = "Failed to initialize MAC support";
                         goto finish;
                 }