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.