Fix invalid access when reinterpret_casting a non-pybind11 PyObject* to instance* (found by Valgrind in #2746) (#2755)

Author: YannickJadoul

include/pybind11/pybind11.h

@@ -504,15 +504,15 @@ class cpp_function : public function {
 
         auto self_value_and_holder = value_and_holder();
         if (overloads->is_constructor) {
-            const auto tinfo = get_type_info((PyTypeObject *) overloads->scope.ptr());
-            const auto pi = reinterpret_cast<instance *>(parent.ptr());
-            self_value_and_holder = pi->get_value_and_holder(tinfo, false);
-
-            if (!self_value_and_holder.type || !self_value_and_holder.inst) {
+            if (!PyObject_TypeCheck(parent.ptr(), (PyTypeObject *) overloads->scope.ptr())) {
                 PyErr_SetString(PyExc_TypeError, "__init__(self, ...) called with invalid `self` argument");
                 return nullptr;
             }
 
+            const auto tinfo = get_type_info((PyTypeObject *) overloads->scope.ptr());
+            const auto pi = reinterpret_cast<instance *>(parent.ptr());
+            self_value_and_holder = pi->get_value_and_holder(tinfo, true);
+
             // If this value is already registered it must mean __init__ is invoked multiple times;
             // we really can't support that in C++, so just ignore the second __init__.
             if (self_value_and_holder.instance_registered())