channel: Fixed potential use-after-free in ssh_channel_get_exit_status().

If ssh_channel_get_exit_status() is called more than once and the
connection closed.
(cherry picked from commit 4e153aed8a)

src/channels.c

@@ -2495,7 +2495,7 @@ int ssh_channel_get_exit_status(ssh_channel channel) {
     return -1;
   }
 
-  while (channel->remote_eof == 0 || channel->exit_status == -1) {
+  while ((channel->remote_eof == 0 || channel->exit_status == -1) && channel->session->alive) {
     /* Parse every incoming packet */
     if (ssh_handle_packets(channel->session,-1) != SSH_OK) {
       return -1;