drakkan/sftpgo: common/common.go; 20 LoC

Found a possible issue in [drakkan/sftpgo](https://www.github.com/drakkan/sftpgo) at [common/common.go](https://github.com/drakkan/sftpgo/blob/a8a17a223a0bc35941366c8b462fa7ceaff9bfdb/common/common.go#L595-L614)

Below is the message reported by the analyzer for this snippet of code. Beware that the analyzer only reports the first issue it finds, so please do not limit your consideration to the contents of the below message.

> range-loop variable c used in defer or goroutine at line 605

[Click here to see the code in its original context.](https://github.com/drakkan/sftpgo/blob/a8a17a223a0bc35941366c8b462fa7ceaff9bfdb/common/common.go#L595-L614)

Click here to show the 20 line(s) of Go which triggered the analyzer.

```go
for _, c := range conns.connections {
idleTime := time.Since(c.GetLastActivity())
isUnauthenticatedFTPUser := (c.GetProtocol() == ProtocolFTP && c.GetUsername() == "")

if idleTime > Config.idleTimeoutAsDuration || (isUnauthenticatedFTPUser && idleTime > Config.idleLoginTimeout) {
defer func(conn ActiveConnection, isFTPNoAuth bool) {
err := conn.Disconnect()
logger.Debug(conn.GetProtocol(), conn.GetID(), "close idle connection, idle time: %v, username: %#v close err: %v",
time.Since(conn.GetLastActivity()), conn.GetUsername(), err)
if isFTPNoAuth {
ip := utils.GetIPFromRemoteAddress(c.GetRemoteAddress())
logger.ConnectionFailedLog("", ip, dataprovider.LoginMethodNoAuthTryed, c.GetProtocol(), "client idle")
metrics.AddNoAuthTryed()
AddDefenderEvent(ip, HostEventNoLoginTried)
dataprovider.ExecutePostLoginHook("", dataprovider.LoginMethodNoAuthTryed, ip, c.GetProtocol(),
dataprovider.ErrNoAuthTryed)
}
}(c, isUnauthenticatedFTPUser)
}
}

```

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commit ID: a8a17a223a0bc35941366c8b462fa7ceaff9bfdb