make casts explicit and fixed the type

I think there is a bug in the implementation of the rand test. The variable is called short offset, and assigned a maximum value of 16'000, yet is stored in a singed char.

I admittedly don't quite understand how this is used exactly, so it would be good if someone with more knowledge about this test could take a look. I've also changed the maximum number that can be assigned to short offset, because 24345 + 16000 > 2^15 would result in an overflow in line 97.