if it is not possible to make the sharing Uri, return a placeholder

The receiver of this can at least make some sense of it, versus null or a
blank string.  This prevents crashes where FDroid.repo.address is not yet
set since the wifi was never enabled or it does not have an IP address yet.

src/org/fdroid/fdroid/Utils.java

@@ -193,6 +193,8 @@ public final class Utils {
     }
 
     public static Uri getSharingUri(Context context, Repo repo) {
+        if (TextUtils.isEmpty(repo.address))
+            return Uri.parse("http://wifi-not-enabled");
         Uri uri = Uri.parse(repo.address.replaceFirst("http", "fdroidrepo"));
         Uri.Builder b = uri.buildUpon();
         b.appendQueryParameter("fingerprint", repo.fingerprint);