ar: Replace one alloca use by xmalloc

ar: Replace one alloca use by xmalloc

This alloca use is inside a lexical block and is used to replace one
element of argv. Use a function local variable, xmalloc and free to
make memory usage pattern more clear.

    * src/ar.c (main): Move newp char pointer declaration up.
    Use xmalloc to allocate space. free at end of main.

Signed-off-by: Mark Wielaard <mark@klomp.org>