Fix channel_get_exit_status bug reported by VicLee

It would return -1 if the channel received the exit status
and the close message at same time.

libssh/channels.c

@@ -1793,7 +1793,9 @@ int channel_get_exit_status(ssh_channel channel) {
       return -1;
     }
     if (channel->open == 0) {
-      return -1;
+      /* When a channel is closed, no exit status message can
+       * come anymore */
+      break;
     }
   }