[fix] fix play and live stat bug when total_nclients is zero.

[plainheart]

I cannot reproduce the case below, so I just have tried to fix play and live stat bug when total_nclients is zero. Check and test it carefully please.
Error case:

{
	"http-flv": {
		"nginx_version": "1.12.2",
		"nginx_http_flv_version": "1.2.6",
		"compiler": "cl 19.13",
		"built": "Mar 15 2019 19:13:20",
		"pid": 11764,
		"uptime": 431483,
		"naccepted": 351,
		"bw_in": 0,
		"bytes_in": 15144154714,
		"bw_out": 0,
		"bytes_out": 15000743868,
		"servers": [{
			"applications": [{
				"name": "live",
				"live": {
					"streams": [{
						"name": "13713500022@channel_1",
						"time": 4537565,
						"bw_in": 0,
						"bytes_in": 0,
						"bw_out": 0,
						"bytes_out": 0,
						"bw_audio": 0,
						"bw_video": 0,
						"clients": [],
						"nclients": 0,
						"publishing": false,
						"active": false
					} {
						"name": "13713500022@channel_2",
						"time": 4537564,
						"bw_in": 0,
						"bytes_in": 0,
						"bw_out": 0,
						"bytes_out": 0,
						"bw_audio": 0,
						"bw_video": 0,
						"clients": [],
						"nclients": 0,
						"publishing": false,
						"active": false
					} {
						"name": "13713500022@channel_3",
						"time": 4537561,
						"bw_in": 0,
						"bytes_in": 0,
						"bw_out": 0,
						"bytes_out": 0,
						"bw_audio": 0,
						"bw_video": 0,
						"clients": [],
						"nclients": 0,
						"publishing": false,
						"active": false
					} {
						"name": "13713500022@channel_4",
						"time": 62929406,
						"bw_in": 0,
						"bytes_in": 0,
						"bw_out": 0,
						"bytes_out": 0,
						"bw_audio": 0,
						"bw_video": 0,
						"clients": [],
						"nclients": 0,
						"publishing": false,
						"active": false
					}],
					"nclients": 0
				}
			}, {
				"name": "test",
				"live": {
					"streams": [],
					"nclients": 0
				}
			}]
		}]
	}
}

[winshining]

@plainheart Thanks! I will check it later.

[winshining]

@plainheart I don't know how to reproduce the fucking bug neither, hope it works.