bug#65726: 29.1.50; Crash in regexp engine

[Mattias Engdegård]

Thank you for fixing it, but now regex-emacs-tests fail with a regexp stack 
overflow. It seems that by fixing this bug, we've unfixed the one that the 
broken code was supposed to fix.

And we should probably include a regression test for this bug as well. The 
previously mentioned

  (string-match (rx (* "a") (* (* "b"))) "a")

might suffice, but extending it to

  (string-match (rx (* "a") (* (or "c" (* "b")))) "a")

is safer in case the regexp compiler decides to simplify (* (* X)) -> (* X).