Marshall Clow found some divide-by-zero warnings with UBSan in rand's binomial_distribution test.  This eliminates the divide-by-zeros and describes in comments the numerical difficulties the test is having.  Each of the problematic tests are exploring edge cases of the distribution.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@177826 91177308-0d34-0410-b5e6-96231b3b80d8