another problem reported by Oleg Paraschenko on the same code in #127450

* libxslt/keys.c: another problem reported by Oleg Paraschenko
  on the same code in #127450
* tests/docs/Makefile.am tests/docs/bug-135.xml
  tests/general/Makefile.am tests/docs/bug-135*: added tests
  to the regression suite for bug #127450.
Daniel