bug#33830: 27.0.50; help-xref-stack contains bad entries

[Alex Branham]

Sometimes `help-xref-stack' contains "bad" entries, which makes
help-go-forward and help-go-back error in a strange way. To see what I
mean:

emacs -q

C-h f car RET
C-h k C-x t
C-h f cdr RET

Now that the *Help* buffer is displaying `cdr', if you try to go back
(with "l" or clicking the back button), you get "user-error C-x t is
undefined".

Alex