According to the tls documentation: tls_read() and tls_write() can
return TLS_WANT_POLLIN and TLS_WANT_POLLOUT which indicates that the
same operation must be repeated immediately.

This commit prevents the libtls backend from failing when libtls returns
TLS_WANT_POLLIN or TLS_WANT_POLLOUT with the following error:

    [tls @ 0x7f6e20005a00] (null)
Signed-off-by: Marton Balint <cus@passwd.hu>