I added some tests to check the ObjectNullReferenceException when the RNG was seeded (GH-921). In debug build, the code is checked so this code can randomly fail with an overflow exception. There weren't any tests for this code before and it's random so I didn't spot it yesterday.

There are three places where this occurs. I've added unit tests that generate 100 random numbers to check this is fixed now. Also, I set the seed to a number that would otherwise overflow for reproducibility purposes.

TestZigguratExponentialGenerator_Unseeded_NeverOverflows()
TestZigguratGenerator_Unseeded_NeverOverflows()