Calling `throw dst_end_of_data_exception()` when the desired number of cycles has been reached means that the fst reader can't tidy up after itself and leads to memory leaks.
This doesn't happen when the `-stop` flag is used because the `Yosys::FstData` struct tracks the end time and skips the outer callback if the simulation has gone past the desired end time.
Move cycle checking into the inner callback along with the time checking means that the outer callback no longer needs to throw an exception in order to stop checking further values, while still allowing the fst reader to finish reading and deallocate memory.
