Fix segfaulting issue with reduce iterator

[lusewell]

Edit is much smaller than diff suggests - mostly just whitespace due to an extra layer of indentation.

The test added generates the segfault described in #381 without the rest of the change, and then the other change fixes this.

[lusewell]

This condition only actualyl needs to be that the last non-zero-strided axis is also the smallest non-zero-strided axis, but this was simpler to write.

[rdbisme]

Hello I rebased this to try to merge it but the test segfaults in the CI...

[lusewell]

Removing the change to iterators.h, the tests still cause a segfault - ie current state on master is broken.

The change of reverting the previous release means its quite a lot of effort to work out what the correct fix would now be.

[lusewell]

EG you've reverted a load of bugfixes such as a8dbfdc, and removed the tests that got added for them...

Why? Can you readd all of these please?

[rdbisme]

Hello, the reason is explained here #388 (comment) . We needed to release some fixes and the state of the main branch was inconsistent.

The previous state is in stored in develop. Feel free to cherry pick the commit you need to fix this.