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

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
Reviewed by: delphij
Approved by: secteam(delphij)
Differential Revision: https://reviews.freebsd.org/D20686