Panic when sending to disconnected client

[tebeka]

Stack trace:

    panic: read tcp 127.0.0.1:10000->127.0.0.1:60443: read: connection reset by peer
    goroutine 65 [running]:
    main.handler({0x100d4ceb8?, 0x36ea6ae02010?}, 0x36ea6ad800e0)
            /Users/evgeny.denisov/go/pkg/mod/github.com/353solutions/droopy@v0.12.2/cmd/droopy/main.go:47 +0x1e0
    created by main.sockListener in goroutine 35
            /Users/evgeny.denisov/go/pkg/mod/github.com/353solutions/droopy@v0.12.2/cmd/droopy/main.go:33 +0x50

While playing

    SEND D3
    WAIT A2
    SEND D2
    WAIT C3
    SEND P1
    SEND P4
    SEND D2
    SEND U3

Client crashed in the middle.

Report by: Evgeny Denisov