fix: connect to stdout fd after starting an ssh shell

this avoids a race condition that would happen if the goroutine tried to read a
file descriptor that wasn't open properly and would lose frame delivery

agent/internal/sshutil/connect.go

@@ -120,9 +120,6 @@ func StartWebSSH(agent *tsnet.TSAgent, params SSHConnectPayload) {
 			}
 		}()
 
-		// This spawns 2 goroutins for stdout and stderr
-		dispatchSSHStdout(params.SessionId, ctx.Stdout, ctx.Stderr)
-
 		// Spin up a shell and wait for the pty to terminate
 		err = sess.Shell()
 		if err != nil {
@@ -131,6 +128,9 @@ func StartWebSSH(agent *tsnet.TSAgent, params SSHConnectPayload) {
 			return
 		}
 
+		// This spawns 2 goroutins for stdout and stderr
+		dispatchSSHStdout(params.SessionId, ctx.Stdout, ctx.Stderr)
+
 		log.Info("Opened an SSH PTY for %s", params.SessionId)
 		sess.Wait()
 		sess.Close()