Fix negative city population on load

When cities are loaded, their buildings do not exist yet and therefore do not contribute to total citizens.
This caused an overflow during target_cities weighted vector re-calculation during load.

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@11890 8aca7d54-2c30-db11-9de9-000461428c89

Author: ceeac

src/simutrans/world/simcity.cc

@@ -1832,9 +1832,11 @@ void stadt_t::step_passagiere()
 	if (step_count >= buildings.get_count()) {
 		step_count = 0;
 	}
+
 	if (buildings.empty()) {
 		return;
 	}
+
 	const gebaeude_t* gb = buildings[step_count];
 
 	// since now backtravels occur, we damp the numbers a little

src/simutrans/world/simcity.h

@@ -481,7 +481,14 @@ class stadt_t
 	/**
 	 * ermittelt die Einwohnerzahl der City
 	 */
-	sint32 get_einwohner() const {return (buildings.get_sum_weight()*6)+((2*bev-arb-won)>>1);}
+	sint32 get_einwohner() const
+	{
+		const sint32 pop_from_buildings = 6 * (sint32)buildings.get_sum_weight();
+		const sint32 pop_from_citizens  = (2 * bev - arb - won) / 2;
+		const sint32 pop = pop_from_buildings + pop_from_citizens;
+
+		return std::max(pop, 0);
+	}
 
 	uint32 get_buildings()  const { return buildings.get_count(); }
 	sint32 get_unemployed() const { return bev - arb; }