fix dangling thread state issue (#5870)

* fix dangling thread state issue

* formatting rules

* use tstate .set(nullptr) to pass clang-tidy check

* fix spelling mistake

* improve comments for maintainability

Author: daltairwalter

include/pybind11/detail/internals.h

@@ -286,8 +286,8 @@ struct internals {
     internals()
         : static_property_type(make_static_property_type()),
           default_metaclass(make_default_metaclass()) {
+        tstate.set(nullptr); // See PR #5870
         PyThreadState *cur_tstate = PyThreadState_Get();
-        tstate = cur_tstate;
 
         istate = cur_tstate->interp;
         registered_exception_translators.push_front(&translate_exception);

include/pybind11/subinterpreter.h

@@ -271,7 +271,7 @@ inline subinterpreter_scoped_activate::subinterpreter_scoped_activate(subinterpr
     // make the interpreter active and acquire the GIL
     old_tstate_ = PyThreadState_Swap(tstate_);
 
-    // save this in internals for scoped_gil calls
+    // save this in internals for scoped_gil calls (see also: PR #5870)
     detail::get_internals().tstate = tstate_;
 }