Initializing PyModuleDef object with PyModuleDef_HEAD_INIT.

Python 3.8 documentation: m_base - Always initialize this member to PyModuleDef_HEAD_INIT.

Long-standing (since first github commit in 2015), inconsequential bug.

Also removing inconsequential Py_INCREF(def): PyModule_Create() resets the reference count to 1.

Author: rwgk

include/pybind11/pybind11.h

@@ -830,11 +830,17 @@ class module : public object {
 #if PY_MAJOR_VERSION >= 3
     explicit module(const char *name, const char *doc = nullptr, PyModuleDef *def = nullptr) {
         if (!def) def = new PyModuleDef();
-        std::memset(def, 0, sizeof(PyModuleDef));
-        def->m_name = name;
-        def->m_doc = options::show_user_defined_docstrings() ? doc : nullptr;
-        def->m_size = -1;
-        Py_INCREF(def);
+        new (def) PyModuleDef {  // Placement new (not an allocation).
+            /* m_base */     PyModuleDef_HEAD_INIT,
+            /* m_name */     name,
+            /* m_doc */      options::show_user_defined_docstrings() ? doc : nullptr,
+            /* m_size */     -1,
+            /* m_methods */  NULL,
+            /* m_slots */    NULL,
+            /* m_traverse */ NULL,
+            /* m_clear */    NULL,
+            /* m_free */     NULL
+        };
         m_ptr = PyModule_Create(def);
 #else
     explicit module(const char *name, const char *doc = nullptr) {