random(6): Fix off-by-one error

[freqlabs]

Summary

After rS355693 random -f sometimes fail to output all the lines of the input file. This is because the range from which random indices are chosen is too big, so occasionally the random selection doesn't correspond to any line and nothing gets printed.

Sponsored by: iXsystems, Inc.

Test Plan

for _ in $(seq 10); do
    seq 0 1 | random -f -
done

Before:

1
2
1
0
2
1
2
1
2
1

After:

2
2
2
2
2
2
2
2
2
2

[freqlabs]

@cem Will you commit this for me? I don't have the bit. Thanks!

[cem]

Yep no problem. Thanks for the report and patch!