Fix extremely rare pmap corruption

c4dcb37de4 added a kill_part without a return 1.

Reproduce with

	sim.loadSave(3062273, 1)
	sim.ensureDeterminism(true)
	tpt.setfpscap(2)
	local function F()
		print(sim.framerender(), sim.randomseed())
		if sim.framerender() == 0 then
			local a = 2247503365
			print(sim.hash(), a)
			sim.framerender(1200)
			sim.reloadSave()
			sim.randomseed(a, a + 1, a + 2, a + 3)
			tpt.set_pause(0)
		end
	end
	event.register(event.tick, F)

src/simulation/elements/PROT.cpp

@@ -108,7 +108,10 @@ static int update(UPDATE_FUNC_ARGS)
 		if (parts[i].life)
 		{
 			if (!--parts[i].life)
+			{
 				sim->kill_part(i);
+				return 1;
+			}
 		}
 		break;
 	default: