deregister: Robustify check, just in case C++ recycles memory with unique_ptr

Author: EricCousineau-TRI

include/pybind11/detail/class.h

@@ -222,7 +222,7 @@ inline bool deregister_instance_impl(void *ptr, instance *self) {
     auto &registered_instances = get_internals().registered_instances;
     auto range = registered_instances.equal_range(ptr);
     for (auto it = range.first; it != range.second; ++it) {
-        if (Py_TYPE(self) == Py_TYPE(it->second)) {
+        if (self == it->second && Py_TYPE(self) == Py_TYPE(it->second)) {
             registered_instances.erase(it);
             return true;
         }