Fix inferior_thread attribute in new_thread event

Commit 72ee03ff58 fixed a use-after-move bug in add_thread_object, but
it changed the inferior_thread attribute to contain the inferior instead
of the actual thread.
This now uses the thread_obj in its new location instead.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28429

gdb/python/py-inferior.c

@@ -333,7 +333,8 @@ add_thread_object (struct thread_info *tp)
     return;
 
   gdbpy_ref<> event = create_thread_event_object (&new_thread_event_object_type,
-						  (PyObject *) inf_obj);
+						  (PyObject *)
+						  entry->thread_obj.get ());
   if (event == NULL
       || evpy_emit_event (event.get (), gdb_py_events.new_thread) < 0)
     gdbpy_print_stack ();

gdb/testsuite/gdb.python/py-infthread.exp

@@ -31,7 +31,8 @@ gdb_test_multiline "install new_thread event handler" \
     "python" "" \
     "seen_a_thread = False" "" \
     "def thread_handler(evt):" "" \
-    "  if evt.inferior_thread is not None:" "" \
+    "  if (evt.inferior_thread is not None" "" \
+    "      and isinstance (evt.inferior_thread, gdb.InferiorThread)):" "" \
     "    global seen_a_thread" "" \
     "    seen_a_thread = True" "" \
     "gdb.events.new_thread.connect(thread_handler)" "" \