[Freeciv-Dev] (PR#2280) Transfer or city with allied units inside --> core dump

["Davide Pagnin via RT" <rt@xxxxxxxxxxxxxx>]

There have been 2 core in beta civserver recently.

The core is raised by a failed sanity check, in particular it is
actually possible to units of not allied players inside a city, and this
will raise an assert.

The fault condition happen cause of transfer city code:

Player A in allied with Player B
Player B unit is in Player A city Example
Player B is not allied with Player C
Player A meet Player C and agree to transfer city Example to Player C

At this point, the unit of Player B is still inside city Example.

THIS IS WRONG! (should be transferred to the nearest city of player B)

I will try to fix this.