Fix NPE from URL->URI conversion

new URL(null) throws MUE
new URI(null) throws NPE

apps/i2psnark/java/src/org/klomp/snark/TrackerClient.java

@@ -875,11 +875,13 @@ public class TrackerClient implements Runnable {
   }
 
   /**
-   *  @param ann an announce URL
+   *  @param ann an announce URL, may be null, returns false if null
    *  @return true for i2p hosts only
    *  @since 0.7.12
    */
   public static boolean isValidAnnounce(String ann) {
+    if (ann == null)
+        return false;
     URI url;
     try {
         url = new URI(ann);