channels: Avoid returning SSH_AGAIN from ssh_channel_poll_timeout()

This addresses a regression introduced in 3bad0607, partially fixed in 022409e9,
but the function was still able to return SSH_AGAIN, which was not expected by
callers.

Based on discussion in [1] and [2]

[1] https://gitlab.com/libssh/libssh-mirror/-/merge_requests/101
[2] https://www.libssh.org/archive/libssh/2020-03/0000029.html

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>

src/channels.c

@@ -3116,6 +3116,12 @@ int ssh_channel_poll_timeout(ssh_channel channel, int timeout, int is_stderr)
         session->session_state == SSH_SESSION_STATE_ERROR) {
         rc = SSH_ERROR;
         goto out;
+    } else if (rc == SSH_AGAIN) {
+        /* If the above timeout expired, it is ok and we do not need to
+         * attempt to check the read buffer. The calling functions do not
+         * expect us to return SSH_AGAIN either here. */
+        rc = SSH_OK;
+        goto out;
     }
     len = ssh_buffer_get_len(stdbuf);
     if (len > 0) {