Exit while socks5 proxy alive causes panic

[ghost-ng]

Describe the bug
Sometimes, the program will crash if you kill a session that has an active socks5 proxy working

To Reproduce
Get session
Start socks5
exit (not close)

Expected behavior
Stop the socks proxy, exit the program

./sliver-server_linux version
v1.5.39 - af46878

[!] Active session disconnected

[server] sliver > 
[server] sliver > exipanic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0xb8 pc=0xf51776]

goroutine 2753 [running]:
github.com/bishopfox/sliver/server/rpc.(*Server).SocksProxy.func2()
	github.com/bishopfox/sliver/server/rpc/rpc-socks.go:139 +0x76
created by github.com/bishopfox/sliver/server/rpc.(*Server).SocksProxy
	github.com/bishopfox/sliver/server/rpc/rpc-socks.go:131 +0x45