channels: Fix ssh_channel_poll_timeout() not returing available bytes

Fixes T211

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>
(cherry picked from commit 022409e99c)

src/channels.c

@@ -3119,6 +3119,11 @@ int ssh_channel_poll_timeout(ssh_channel channel, int timeout, int is_stderr)
     }
     len = ssh_buffer_get_len(stdbuf);
     if (len > 0) {
+        if (len > INT_MAX) {
+            rc = SSH_ERROR;
+        } else {
+            rc = (int)len;
+        }
         goto out;
     }
     if (channel->remote_eof) {