Ensure next_index available when loading old stored KeyedVectors models

[gojomo]

Fix #3114.

[mpenkov]

Do we want to add some unit tests here, to validate the solution and prevent regressions?

[gojomo]

Do we want to add some unit tests here, to validate the solution and prevent regressions?

Yes, per my comment on #3114: #3114 (comment) - but not sure when I'd be able to get to it, so just dropped this 1st quick stab at a maybe-fix here, as a potential building block for others.

[piskvorky]

Looks like the first and the last stab :)

Do we merge as-is, or keep waiting for someone to continue? Or close. @gojomo @mpenkov

[gojomo]

I think this is sufficiently straightforward & low-risk it can be added without a unit test. We still have giant gaps in rigorous/complete testing of backward-compatibility goals – a well-labeled, growing library of older models whose loadability into newer versions is automatically checked has been discussed, & (once such processes 1st established) wouldn't be too hard to maintain, & would have caught this (& other) issues earlier. (See related ideas in #2967.) But getting that started (& ideally then using the improved rigor to clean out all the junk test files no longer relevant) seems a bigger task than anyone is currently able to prioritize.