Fix wrong parse of macro parameter and crash

Fix wrong parse of macro parameter and crash

Copy-pasting $#\n$ from text to LyX gives the error message:

    MathMacroArgument::MathMacroArgument: wrong Argument id

and it is not hard to get a crash soon after.

There are legitimate uses of # not followed by 1..9 in LaTeX and it is good to
parse them correctly when importing from LaTeX.