core: make unit_free() accept NULL pointers

We generally try to make our destructors robust regarding NULL pointers, much
in the same way as glibc's free(). Do this also for unit_free().

Follow-up for #4748.

src/core/device.c

@@ -359,7 +359,7 @@ static int device_setup_unit(Manager *m, struct udev_device *dev, const char *pa
 fail:
         log_unit_warning_errno(u, r, "Failed to set up device unit: %m");
 
-        if (delete && u)
+        if (delete)
                 unit_free(u);
 
         return r;

src/core/mount.c

@@ -1509,7 +1509,7 @@ static int mount_setup_unit(
 fail:
         log_warning_errno(r, "Failed to set up mount unit: %m");
 
-        if (delete && u)
+        if (delete)
                 unit_free(u);
 
         return r;

src/core/swap.c

@@ -420,7 +420,7 @@ static int swap_setup_unit(
 fail:
         log_unit_warning_errno(u, r, "Failed to load swap unit: %m");
 
-        if (delete && u)
+        if (delete)
                 unit_free(u);
 
         return r;

src/core/unit.c

@@ -516,7 +516,8 @@ void unit_free(Unit *u) {
         Iterator i;
         char *t;
 
-        assert(u);
+        if (!u)
+                return;
 
         if (u->transient_file)
                 fclose(u->transient_file);