Move an align_safe group to get robust tl_replace functions

The \tl_if_in:nnTF conditional breaks when used in an halign
with a second argument containing |&|.  Thus it is best in
\tl_replace_all/once:Nnn to issue the \group_align_safe_begin:
command as early as possible.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5321 de43f980-851b-0410-b2f7-c40aca1f87e0