Handle buggy situations with SSL_ERROR_SYSCALL

Handle buggy situations with SSL_ERROR_SYSCALL

See "BUGS" section at:

https://www.openssl.org/docs/man1.1.1/man3/SSL_get_error.html

It is mentioned there that when TLS status equals SSL_ERROR_SYSCALL
AND errno == 0 it means that underlying transport layer returned EOF
prematurely.  However, we are managing the transport ourselves, so we
should just resume reading from the TCP socket.

It seems that this case has been handled properly on modern versions
of OpenSSL. That being said, the situation goes in line with the
manual: it is briefly mentioned there that SSL_ERROR_SYSCALL might be
returned not only in a case of low-level errors (like system call
failures).