fix a potential race condition for pre-login and ext auth

hooks

doing something like this:

err = provider.updateUser(u)
...
return provider.userExists(username)

could be racy if another update happen before

provider.userExists(username)

also pass a pointer to updateUser so if the user is modified inside
"validateUser" we can just return the modified user without do a new
query

sftpd/ssh_cmd.go

@@ -537,7 +537,7 @@ func (c *sshCommand) getCopyPaths() (string, string, error) {
 	if strings.HasSuffix(sshDestPath, "/") {
 		sshDestPath = path.Join(sshDestPath, path.Base(sshSourcePath))
 	}
-	if len(sshSourcePath) == 0 || len(sshDestPath) == 0 || len(c.args) != 2 {
+	if sshSourcePath == "" || sshDestPath == "" || len(c.args) != 2 {
 		err := errors.New("usage sftpgo-copy <source dir path> <destination dir path>")
 		return "", "", err
 	}
@@ -606,7 +606,7 @@ func (c *sshCommand) checkCopyPermissions(fsSourcePath, fsDestPath, sshSourcePat
 
 func (c *sshCommand) getRemovePath() (string, error) {
 	sshDestPath := c.getDestPath()
-	if len(sshDestPath) == 0 || len(c.args) != 1 {
+	if sshDestPath == "" || len(c.args) != 1 {
 		err := errors.New("usage sftpgo-remove <destination path>")
 		return "", err
 	}