random(4): Fix a regression in short AES mode reads

[cem]

Summary

In r349154, random device reads of size < 16 bytes (AES block size) were
accidentally broken to loop forever. Correct the loop condition for small
reads.

Reported by: pho

Test Plan

$ dd if=/dev/random bs=1 count=1 of=/dev/null
...
(should exit promptly)