manager: let manager_free() handle NULLs

This makes the calling code a bit simpler.

src/core/main.c

@@ -1799,10 +1799,7 @@ int main(int argc, char *argv[]) {
 finish:
         pager_close();
 
-        if (m) {
-                manager_free(m);
-                m = NULL;
-        }
+        m = manager_free(m);
 
         for (j = 0; j < ELEMENTSOF(arg_default_rlimit); j++) {
                 free(arg_default_rlimit[j]);

src/core/manager.c

@@ -889,11 +889,12 @@ static void manager_clear_jobs_and_units(Manager *m) {
         m->n_running_jobs = 0;
 }
 
-void manager_free(Manager *m) {
+Manager* manager_free(Manager *m) {
         UnitType c;
         int i;
 
-        assert(m);
+        if (!m)
+                return NULL;
 
         manager_clear_jobs_and_units(m);
 
@@ -955,6 +956,7 @@ void manager_free(Manager *m) {
         hashmap_free(m->units_requiring_mounts_for);
 
         free(m);
+        return NULL;
 }
 
 int manager_enumerate(Manager *m) {

src/core/manager.h

@@ -296,7 +296,7 @@ struct Manager {
 };
 
 int manager_new(SystemdRunningAs running_as, bool test_run, Manager **m);
-void manager_free(Manager *m);
+Manager* manager_free(Manager *m);
 
 int manager_enumerate(Manager *m);
 int manager_startup(Manager *m, FILE *serialization, FDSet *fds);