Fix actual undefined behavior exposed by previous changes.

It turns out the previous commit message is incorrect, the `inst` pointer is actually dereferenced, in the `value_and_holder` ctor here:

https://github.com/pybind/pybind11/blob/f3e0602802c7840992c97f4960515777cad6a5c7/include/pybind11/detail/type_caster_base.h#L262-L263

```
259     // Main constructor for a found value/holder:
260     value_and_holder(instance *i, const detail::type_info *type, size_t vpos, size_t index)
261         : inst{i}, index{index}, type{type},
262           vh{inst->simple_layout ? inst->simple_value_holder
263                                  : &inst->nonsimple.values_and_holders[vpos]} {}
```

include/pybind11/detail/type_caster_base.h

@@ -349,12 +349,16 @@ struct values_and_holders {
         const type_vec *types = nullptr;
         value_and_holder curr;
         friend struct values_and_holders;
-        iterator(instance *inst, const type_vec *tinfo)
-            : inst{inst}, types{tinfo},
-              curr(inst /* instance */,
-                   types->empty() ? nullptr : (*types)[0] /* type info */,
-                   0, /* vpos: (non-simple types only): the first vptr comes first */
-                   0 /* index */) {}
+        iterator(instance *inst, const type_vec *tinfo) : inst{inst}, types{tinfo} {
+            if (inst != nullptr) {
+                assert(!types->empty());
+                curr = value_and_holder(
+                    inst /* instance */,
+                    (*types)[0] /* type info */,
+                    0, /* vpos: (non-simple types only): the first vptr comes first */
+                    0 /* index */);
+            }
+        }
         // Past-the-end iterator:
         explicit iterator(size_t end) : curr(end) {}