Fix bug with \x{....} in XeTeX/LuaTeX.

I don't know how to test this automatically, but I've
tested it manually.  The problem occured when \x{...}
was used in a regular expression, with an argument
bigger than hexadecimal FF.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4455 de43f980-851b-0410-b2f7-c40aca1f87e0