Fix buffer overflow in exportGraphics

src/universe/universe.cpp

@@ -1167,7 +1167,7 @@ void cUniverse::exportGraphics() {
 		}
 	}
 	for(mon_num_t monst : party.imprisoned_monst) {
-		if(monst != 0)
+		if(monst > 0 && monst < scenario.scen_monsters.size())
 			check_monst(scenario.scen_monsters[monst]);
 	}
 	// And then, just add all the graphics, and update references to them