Experiment: apply 41c37a3 also in smart_holder_from_unique_ptr()

Author: rwgk

include/pybind11/detail/type_caster_base.h

@@ -588,7 +588,8 @@ handle smart_holder_from_unique_ptr(std::unique_ptr<T, D> &&src,
     auto *inst_raw_ptr = reinterpret_cast<instance *>(inst.ptr());
     inst_raw_ptr->owned = true;
     void *&valueptr = values_and_holders(inst_raw_ptr).begin()->value_ptr();
-    valueptr = src_raw_void_ptr;
+    if (valueptr) {
+    }
 
     if (static_cast<void *>(src.get()) == src_raw_void_ptr) {
         // This is a multiple-inheritance situation that is incompatible with the current