真机上可以正常运行，在多款模拟器运行进程会被杀

[rwz657026189]

在模拟器（Nexus 6P API 27(Android 8.1.0,API 27)）上运行一会之后，vpnservice会被杀，期间网络不能正常访问。已测试多款模拟器，都出现该情况。但真机实测是正常的。出现的崩溃日志如下：

11-13 01:00:37.957 8316-0/com.package.test:V2RayVPN E/Go: (0x0,0x0)
11-13 01:00:37.957 8316-8354/com.package.test:V2RayVPN E/GoLog: (0x0,0x0)
11-13 01:00:37.961 8316-0/com.package.test:V2RayVPN E/Go: panic: runtime error: invalid memory address or nil pointer dereference
11-13 01:00:37.962 8316-0/com.package.test:V2RayVPN E/Go: [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xccdd50f7]
goroutine 15 [running]:
os.(*Process).signal(0x0, 0xcd612170, 0xcda3f034, 0x0, 0x0)
11-13 01:00:37.963 8316-0/com.package.test:V2RayVPN E/Go: /usr/local/go/src/os/exec_unix.go:56 +0x27
os.(*Process).Signal(0x0, 0xcd612170, 0xcda3f034, 0x9c49c000, 0x2)
11-13 01:00:37.966 8316-0/com.package.test:V2RayVPN E/Go: /usr/local/go/src/os/exec.go:131 +0x2b
11-13 01:00:37.967 8316-0/com.package.test:V2RayVPN E/Go: os.(*Process).kill(0x0, 0x0, 0x0)
/usr/local/go/src/os/exec_posix.go:54 +0x33
11-13 01:00:37.967 8316-0/com.package.test:V2RayVPN E/Go: os.(*Process).Kill(0x0, 0x1, 0x1)
11-13 01:00:37.968 8316-0/com.package.test:V2RayVPN E/Go: /usr/local/go/src/os/exec.go:116 +0x1b
11-13 01:00:37.968 8316-8354/com.package.test:V2RayVPN E/GoLog: panic: runtime error: invalid memory address or nil pointer dereference
11-13 01:00:37.968 8316-0/com.package.test:V2RayVPN E/Go: github.com/xiaokangwang/AndroidLibV2ray/Process/Escort.(*Escorting).EscortingDown(0x9c5e6040)
11-13 01:00:37.969 8316-0/com.package.test:V2RayVPN E/Go: /Users/uimeet/go/src/github.com/xiaokangwang/AndroidLibV2ray/Process/Escort/escort.go:104 +0x8b
github.com/xiaokangwang/AndroidLibV2ray/VPN.(*VPNSupport).VpnShutdown(0x9c49a840)
11-13 01:00:37.971 8316-0/com.package.test:V2RayVPN E/Go: /Users/uimeet/go/src/github.com/xiaokangwang/AndroidLibV2ray/VPN/vpnservice_interact.go:74 +0xb3
github.com/xiaokangwang/AndroidLibV2ray.(*V2RayPoint).stopLoopW(0x9c4bc230)
11-13 01:00:37.972 8316-0/com.package.test:V2RayVPN E/Go: /Users/uimeet/go/src/github.com/xiaokangwang/AndroidLibV2ray/interact.go:259 +0x82
created by github.com/xiaokangwang/AndroidLibV2ray.(*V2RayPoint).StopLoop
/Users/uimeet/go/src/github.com/xiaokangwang/AndroidLibV2ray/interact.go:279 +0x59

--------- beginning of crash

11-13 01:00:37.972 8316-8360/com.package.test:V2RayVPN A/libc: Fatal signal 6 (SIGABRT), code -6 in tid 8360 (Thread-9), pid 8316 (.tunan:V2RayVPN)