Commit ffda5db6 ("io/channel-tls: fix handling of bigger read buffers")
changed the behavior of the TLS io channels to schedule a second reading
attempt if there is still incoming data pending. This caused a regression
with backends like the sclpconsole that check in their read function that
the sender does not try to write more bytes to it than the device can
currently handle.

The problem can be reproduced like this:

 1) In one terminal, do this:

  mkdir qemu-pki
  cd qemu-pki
  openssl genrsa 2048 > ca-key.pem
  openssl req -new -x509 -nodes -days 365000 -key ca-key.pem -out ca-cert.pem
  # enter some dummy value for the cert
  openssl genrsa 2048 > server-key.pem
  openssl req -new -x509 -nodes -days 365000 -key server-key.pem \
    -out server-cert.pem
  # enter some other dummy values for the cert

  gnutls-serv --echo --x509cafile ca-cert.pem --x509keyfile server-key.pem \
              --x509certfile server-cert.pem ...