Base: Fixed shift amount in genrand() to work with large numbers

Base: Fixed shift amount in genrand() to work with large numbers

The previous version didn't work correctly with max integer values (2^31-1 for
32-bit, 2^63-1 for 64bit version), causing "shift" to become -1.  For smaller
numbers, it wouldn't have caused functional errors, but would have resulted in
more than necessary loops in the while loop.  Special-cased cases when (max + 1
== 0) to prevent the ceilLog2 functions from failing.