xcor.py, --random-source fails if file is too small

possible fix:

Make the file bigger so that --random-source can work

tags2 = 'cat.tagAlign.gz'
out, err = utils.run_pipe([
    "zcat %s %s %s" %
    (tag, tag, tag)
], outfile=tags2)

steps = [
    'zcat %s' % (tag),
    'grep -v "chrM"',
    'shuf -n %d --random-source=%s' % (NREADS, tags2)]