avoid NPE on hello message with missing "yourip" key

http://mantis.tokeek.de/view.php?id=684

source/net/yacy/peers/Protocol.java

@@ -259,6 +259,10 @@ public final class Protocol {
             mySeed.setIPs(Switchboard.getSwitchboard().myPublicIPs());
         } else {
             final String myIP = result.get("yourip");
+            if (myIP == null) {
+                Network.log.info("yacyClient.hello result error: Peer sent incompleet hello message (key yourip is missing)");
+                return null; // no success
+            }
             // with the IPv6 extension, this may contain several ips, separated by comma ','
             HashSet<String> h = new HashSet<>();
             for (String s: CommonPattern.COMMA.split(myIP)) {