Fix entity related bugs

* Make minetest.add_entity() binary-safe
* Fix on_death pushing dummy ObjectRef instead of nil

src/script/cpp_api/s_entity.cpp

@@ -244,7 +244,7 @@ bool ScriptApiEntity::luaentity_Punch(u16 id,
 {
 	SCRIPTAPI_PRECHECKHEADER
 
-	//infostream<<"scriptapi_luaentity_step: id="<<id<<std::endl;
+	assert(puncher);
 
 	int error_handler = PUSH_ERROR_HANDLER(L);
 
@@ -294,7 +294,10 @@ bool ScriptApiEntity::luaentity_run_simple_callback(u16 id,
 	}
 	luaL_checktype(L, -1, LUA_TFUNCTION);
 	lua_pushvalue(L, object);  // self
-	objectrefGetOrCreate(L, sao);  // killer reference
+	if (sao)
+		objectrefGetOrCreate(L, sao);  // sao reference
+	else
+		lua_pushnil(L);
 
 	setOriginFromTable(object);
 	PCALL_RES(lua_pcall(L, 2, 1, error_handler));

src/script/lua_api/l_env.cpp

@@ -640,7 +640,7 @@ int ModApiEnvMod::l_add_entity(lua_State *L)
 
 	v3f pos = checkFloatPos(L, 1);
 	const char *name = luaL_checkstring(L, 2);
-	const char *staticdata = luaL_optstring(L, 3, "");
+	std::string staticdata = readParam<std::string>(L, 3, "");
 
 	ServerActiveObject *obj = new LuaEntitySAO(env, pos, name, staticdata);
 	int objectid = env->addActiveObject(obj);