unit_is_bound_by_inactive: fix return pointer check

*ret_culprit should be set if ret_culprit has been passed a non-null value,
checking the previous *ret_culprit value does not make sense.

This would cause the culprit to not properly be assigned, leading to
pid1 crash when a unit could not be stopped.

Fixes: #21476

src/core/unit.c

@@ -2129,7 +2129,7 @@ bool unit_is_bound_by_inactive(Unit *u, Unit **ret_culprit) {
                         continue;
 
                 if (UNIT_IS_INACTIVE_OR_FAILED(unit_active_state(other))) {
-                        if (*ret_culprit)
+                        if (ret_culprit)
                                 *ret_culprit = other;
 
                         return true;