lib/csrand.c: Fix the lower part of the domain of csrand_uniform()

lib/csrand.c: Fix the lower part of the domain of csrand_uniform()

I accidentally broke this code during an un-optimization.  We need to
start from a random value of the width of the limit, that is, 32 bits.

Thanks to Jason for pointing to his similar code in the kernel, which
made me see my mistake.

Fixes: 2a61122b5e8f ("Unoptimize the higher part of the domain of csrand_uniform()")
Closes: <https://github.com/shadow-maint/shadow/issues/1015>
Reported-by: Michael Brunnbauer <https://github.com/michaelbrunnbauer>
Link: <https://git.zx2c4.com/linux-rng/tree/drivers/char/random.c#n535>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Link: <https://github.com/shadow-maint/shadow/pull/638>
Link: <https://github.com/shadow-maint/shadow/issues/634>
Link: <https://github.com/shadow-maint/shadow/pull/624>
Tested-by: Michael Brunnbauer <https://github.com/michaelbrunnbauer>
Reviewed-by: Michael Brunnbauer <https://github.com/michaelbrunnbauer>
Signed-off-by: Alejandro Colomar <alx@kernel.org>