The Barbarian Nationality ID should be 0 and not -1

[rajko-horvat]

Describe the bug
The old Sve/Map save format stores Barbarian Nationality ID as -1, but in Nations array the proper Barbarian Nationality ID is 0.

To Reproduce
Steps to reproduce the behavior:

1. When Barbarian unit conquers the city an Array Out Of Bounds error happens.

Expected behavior
The Barbarian nationality should clearly be displayed when a city is conquered by Barbarians.

OS version(s) affected:

• All