The lowest int generated by StdGen is 1. Fixes ghc #8899.

This follows from the following line in the function stdNext
(z' is the next generated Int):

    z' = if z < 1 then z + 2147483562 else z
Cleanup + add comments

Fixme reverred to formatting of comments, which was done in
24260b389852ab109de6b62822d889d0e66ae723
Merge pull request #7 from thomie/T8899

Fix for ghc ticket #8899: StdGen does not generate 0