ssh commands: improve command ended detection

Sometime we can have this error:

read |0: file already closed

reading from the command standard error, this means that the command is
already finished so we don't need to do nothing.
This happen randomically while running the test cases on travis.

.travis.yml

@@ -5,7 +5,6 @@ os:
   - osx
 
 go:
-  - "1.12.x"
   - "1.13.x"
 
 env:

README.md

@@ -34,7 +34,7 @@ Regularly the test cases are manually executed and pass on Windows. Other UNIX v
 
 ## Requirements
 
-- Go 1.12 or higher as build only dependency.
+- Go 1.13 or higher as build only dependency.
 - A suitable SQL server or key/value store to use as data provider: PostreSQL 9.4+ or MySQL 5.6+ or SQLite 3.x or bbolt 1.3.x
 
 ## Installation

sftpd/ssh_cmd.go

@@ -261,7 +261,8 @@ func (c *sshCommand) executeSystemCommand(command systemCommand) error {
 		w, e := transfer.copyFromReaderToWriter(c.connection.channel.Stderr(), stderr, 0)
 		c.connection.Log(logger.LevelDebug, logSenderSSH, "command: %#v, copy from sdterr to remote command ended, written: %v err: %v",
 			c.connection.command, w, e)
-		if e != nil || w > 0 {
+		// os.ErrClosed means that the command is finished so we don't need to to nothing
+		if (e != nil && !errors.Is(e, os.ErrClosed)) || w > 0 {
 			once.Do(closeCmdOnError)
 		}
 	}()