xkbcomp: Don't crash on no-op modmask expressions

If we have an expression of the form 'l1' in an interp section, we
unconditionally try to dereference its args, even if it has none.

Signed-off-by: Daniel Stone <daniels@collabora.com>
ClearCompatInfo(CompatInfo *info)
{
    free(info->name);
    darray_free(info->interps);
}

ClearCompatInfo(CompatInfo *info)
{
    free(info->name);
    darray_free(info->interps);
}
