preview is smaller than the padded data

When the requested previewed data is smaller than the padded data, it results in the error in BlockSplitter:
```
File "/home/algol/Documents/DEV/httomo/httomo/runner/task_runner.py", line 112, in _execute_section
splitter = BlockSplitter(self.source, section.max_slices)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/algol/Documents/DEV/httomo/httomo/runner/block_split.py", line 27, in __init__
self._num_blocks = math.ceil(self._chunk_size / self._max_slices)
~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
ZeroDivisionError: division by zero
```