Border tile ownership incorrectly set to neutral on city destroyed

[XHawk87]

Describe the bug
When a city is destroyed, tiles around it are incorrectly set to neutral tile ownership, even when another (older) city covers some or all of those tiles. Borders are correctly recalculated at TC, but not before players can take advantage of the situation.

When restrictinfra is set, this allows use of restricted infrastructure on tiles that should belong to an enemy, but are incorrectly set to neutral.

It also allows a player to found a city in what should be enemy territory but is not, allowing them to claim more border tiles than they should be able to.

To Reproduce
Steps to reproduce the behaviour:

1. Build some heavily overlapping cities of at least 2 different nations.
2. Attack and destroy one of the newer cities that overlap with a friendly older city.
3. Notice that the border tile ownership is set to neutral in overlapping areas.
4. Notice that roads can be used in these overlapping neutral tiles
5. Notice that cities can be placed in these overlapping neutral tiles
6. Notice that the border changes at TC

Expected behaviour
Tiles within range of neighbouring cities should remain under their ownership when a city is destroyed, such that no border changes are observed at TC.

[Corbeau-1]

Borders are correctly recalculated at TC

Not entirely true. If the removed city is on the border, it may happen that at TC some of the neutralised tiles change hands, changing the old border.