Fix crash after a dimension change creates many buildings at once

[Flashmyname]

Summary

Switching to a dimension with many buildings filled the building pool, and the next IPL stream-in crashed the client.

Motivation

Regression from #5139.

Test plan

With runcode, a few seconds apart:

crun setElementDimension(localPlayer,1) setElementPosition(localPlayer,-2900,-2900,5)
crun createBuilding(1337,0,0,3) for i=1,engineGetPoolCapacity("building")-engineGetPoolUsedCapacity("building")+100 do createBuilding(1337,0,0,3) end
crun setElementDimension(localPlayer,0) setElementPosition(localPlayer,-2150,200,40) return engineGetPoolCapacity("building")-engineGetPoolUsedCapacity("building")

Master returns 0 and crashes at 0x5381A5 as the area streams in, with the fix it returns the free slot count and keeps running.

Checklist

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