dict_readonly: member init (#1661)

fix missing member initialization in pytypes: read-only dict.

Found with coverity in a downstream project.

include/pybind11/pytypes.h

@@ -708,7 +708,7 @@ protected:
 
 private:
     handle obj;
-    PyObject *key, *value;
+    PyObject *key = nullptr, *value = nullptr;
     ssize_t pos = -1;
 };
 NAMESPACE_END(iterator_policies)