Fix `cond` miscompilation bug

Fix `cond` miscompilation bug

This fixes a bug that miscompiled

  (cond ... C S1...Sn)

where S1...Sn are switch clauses (that can be compiled into a switch
op) and C a non-switch clause, by tucking on an extra copy of C at the
end.  This was a serious wrong-code bug when the condition of C had
side-effects; otherwise it was only a waste of time and space.

* lisp/emacs-lisp/bytecomp.el (byte-compile-cond): Fix.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--test-cases):
Add test case.