RowIterator::prev() should not throw an exception if you go out of bounds

[dkarlovi]

This is:

- [x] a bug report
- [ ] a feature request
- [ ] **not** a usage question (ask them on https://stackoverflow.com/questions/tagged/phpspreadsheet or https://gitter.im/PHPOffice/PhpSpreadsheet)

What is the expected behavior?

RowIterator::prev() not throwing an exception if you go out of bounds.

What is the current behavior?

RowIterator::prev() throwing an exception if you go out of bounds.

What are the steps to reproduce?

I've implemented an RowIterator decorator which allows me to iterate in reverse,
https://github.com/dkarlovi/xls-transmailifier/blob/command/src/Transmailifier/ReverseRowIterator.php#L63

The way current code works, it will throw an exception if you try to go out of bounds:

PhpSpreadsheet/src/PhpSpreadsheet/Worksheet/RowIterator.php

Lines 160 to 162 in 043327b

	if ($this->position <= $this->startRow) {
	throw new PhpSpreadsheetException("Row is already at the beginning of range ({$this->startRow} - {$this->endRow})");
	}

But, how iterators work, they WILL go out of bounds and then call valid(). You can see this by having next() not check if it's out of bounds, it just does.

I believe this to be a bug.

Which versions of PhpSpreadsheet and PHP are affected?

Latest master, as linked.