Fix parse_tls_serverhello() bug

This PR fixes a minor bug in parse_tls_serverhello(). In some cases the server's entire response is not retrieved. In these cases, it is possible that the response from the server ends with a portion of a handshake message.

The loop at the beginning of parse_tls_serverhello() extracts the various handshake and alert messages from the server's response. If it gets to the end of the response, and what is at the end is not a complete message, it should just ignore that fragment and break out of the loop. At the moment, however, parse_tls_serverhello() just continues in the loop rather than breaking out. This has not been a problem up to now, since $msg_len is usually set to a positive value from a previous iteration of the loop, which causes the loop to end.

In the case of the server identified in #1353, however, $msg_len is 0 and so the continue rather than break results in an endless loop.

testssl.sh

@@ -11527,7 +11527,7 @@ parse_tls_serverhello() {
                else
                     # This could just be a result of the server's response being
                     # split across two or more packets.
-                    continue
+                    break
                fi
           fi
           tls_content_type="${tls_hello_ascii:i:2}"