MINOR: ssl: don't abort after sending 16kB

MINOR: ssl: don't abort after sending 16kB

SSL records are 16kB max. When trying to send larger data chunks at once,
SSL_read() only processes 16kB and ssl_sock_from_buf() believes it means
the system buffers are full, which is not the case, contrary to raw_sock.
This is particularly noticeable with HTTP/2 when using a 64kB buffer with
multiple streams, as the mux buffer can start to fill up pretty quickly
in this situation, slowing down the data delivery.