This reverts commit b3f42f9.

Currently, there is an issue with a subtle incompatibility between
different iterators. The old SqshReader expected, that all blocks
(except the last one) are of the same size. This is not true for
SqshFileIterator when iterating over sparse files. The new
SqshReader2 does not have this limitation.

This fixes a bug when the data storage wrongfully points to the iterator
when it actually needs to read from the buffer.

A safeguard `assert()` call has been added to the `reader_fill_buffer()`
to detect this error in case of regression. Also a test case has been
added to reproduce this error.