Fixed a small bug in rdrand.c. There was an integer overflow because the block_count variable
was a 32 bit int, so if 2^32 blocks (=16 GB of data) or more data was requested to be
generated, there would not be enough room in the integer to hold the value. I changed it
to a 64 bit variable (uint_64t). Note that an overflow of the variable is definitely still
possible if you decided to make an enormous file (more than a few petabytes).