Fix overpopulation bug when taking abandoned colony.

[ajkfreelance]

Hoping I can persuade adoption of this approach as it eliminates the
entire class of bugs. It is good practice to encapsulate enforcement of
post-conditions, rather than externalize it to all callers (which also
creates code redundancies and induces linkage/exposure to internals of
other objects, such as Planet).

[ajkfreelance]

Note that this bug can be demonstrated with the same savegame as for #78:

• Pick a system near other and abandon it, e.g. Errai
• Wait a turn for abandonment to happen
• Schedule more than 115 colonists to reclaim it (e.g. from Acubens, Arneb, Sadalsuud)
• Observe overpopulation of the colony on the next turn