Handle edge case in fr_rand_init() and, we suspect, oveflow (CID #1604611) (#5434)

Handle edge case in fr_rand_init() and, we suspect, oveflow  (CID #1604611) (#5434)

To handle the rare case of not filling fr_rand_pool.randrsl in a single read,
adjust the location passed to read() to skip what was read in a previous
interation. This is done in a way consistent with the handling of this case
in 3.x, which should also deal with the overflow_sink complaint from Coverity.