UDIFBlockInputStream.java: Fix overflow in zero/copy block skip methods.

When skipping data beyond the 2^31-1 byte mark, the skip methods for
zero blocks and copy blocks experienced an 'int' overflow causing
negative values to be added to the position (which should never happen
in a skip method).

Fixed by removing the cast to 'int' carried over from its overridden
method. Also added some checks to make sure that we catch such internal
inconsistencies if anything similar comes up in the future.