TLS: clear 'errno' when handling SSL status

Sometimes tls_do_bio() might be called when there is no new data to
process (most notably, when resuming reads), in such a case internal
TLS session state will remain untouched and old value in 'errno' will
alter the result of SSL_get_error() call, possibly making it to return
SSL_ERROR_SYSCALL. This value will be treated as an error, and will
lead to closing the connection, which is not what expected.

lib/isc/netmgr/tlsstream.c

@@ -496,6 +496,7 @@ tls_do_bio(isc_nmsocket_t *sock, isc_region_t *received_data,
 			}
 		}
 	}
+	errno = 0;
 	tls_status = SSL_get_error(sock->tlsstream.tls, rv);
 	saved_errno = errno;