BSecureSocket: fix read error handling

The current documentation
(https://www.openssl.org/docs/man1.1.1/man3/SSL_read.html) says that
SSL_read returning 0 should be handled as an error. So, let's do that.

Change-Id: I6781826ea700b6e597fd2d444fd96a1a5270c6cf
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3620
Reviewed-by: Stephan Aßmus <superstippi@gmx.de>

src/kits/network/libnetapi/SecureSocket.cpp

@@ -540,7 +540,7 @@ BSecureSocket::Read(void* buffer, size_t size)
 	int retry;
 	do {
 		bytesRead = SSL_read(fPrivate->fSSL, buffer, size);
-		if (bytesRead >= 0)
+		if (bytesRead > 0)
 			return bytesRead;
 
 		if (errno != EINTR) {