app_lua: restore top index to lua stack after executing a function

- could be related to GH #1577

(cherry picked from commit eb96593)
(cherry picked from commit e872f70)

src/modules/app_lua/app_lua_api.c

@@ -646,6 +646,7 @@ int app_lua_run_ex(sip_msg_t *msg, char *func, char *p1, char *p2,
 	int ret;
 	str txt;
 	sip_msg_t *bmsg;
+	int ltop;
 
 	if(_sr_L_env.LL==NULL)
 	{
@@ -663,7 +664,8 @@ int app_lua_run_ex(sip_msg_t *msg, char *func, char *p1, char *p2,
 	}
 	else LM_DBG("reload deactivated\n");
 	LM_DBG("executing Lua function: [[%s]]\n", func);
-	LM_DBG("lua top index is: %d\n", lua_gettop(_sr_L_env.LL));
+	ltop = lua_gettop(_sr_L_env.LL);
+	LM_DBG("lua top index is: %d\n", ltop);
 	lua_getglobal(_sr_L_env.LL, func);
 	if(!lua_isfunction(_sr_L_env.LL, -1))
 	{
@@ -674,8 +676,12 @@ int app_lua_run_ex(sip_msg_t *msg, char *func, char *p1, char *p2,
 				lua_typename(_sr_L_env.LL,lua_type(_sr_L_env.LL, -1)));
 			txt.s = (char*)lua_tostring(_sr_L_env.LL, -1);
 			LM_ERR("error from Lua: %s\n", (txt.s)?txt.s:"unknown");
+			/* restores the original stack size */
+			lua_settop(_sr_L_env.LL, ltop);
 			return -1;
 		} else {
+			/* restores the original stack size */
+			lua_settop(_sr_L_env.LL, ltop);
 			return 1;
 		}
 	}
@@ -721,13 +727,20 @@ int app_lua_run_ex(sip_msg_t *msg, char *func, char *p1, char *p2,
 		}
 		lua_pop(_sr_L_env.LL, 1);
 		if(n==1) {
+			/* restores the original stack size */
+			lua_settop(_sr_L_env.LL, ltop);
 			return 1;
 		} else {
 			LM_ERR("error executing: %s (err: %d)\n", func, ret);
+			/* restores the original stack size */
+			lua_settop(_sr_L_env.LL, ltop);
 			return -1;
 		}
 	}
 
+	/* restores the original stack size */
+	lua_settop(_sr_L_env.LL, ltop);
+
 	return 1;
 }