Don't crash trying to show item at negative position (#6407)

I have no idea what code path could pass a negative number there,
but apparently there are users who experience a crash when trying
to show a negative position.

app/src/main/java/de/danoeh/antennapod/fragment/ItemPagerFragment.java

@@ -48,7 +48,7 @@ public static ItemPagerFragment newInstance(long[] feeditems, int feedItemPos) {
         ItemPagerFragment fragment = new ItemPagerFragment();
         Bundle args = new Bundle();
         args.putLongArray(ARG_FEEDITEMS, feeditems);
-        args.putInt(ARG_FEEDITEM_POS, feedItemPos);
+        args.putInt(ARG_FEEDITEM_POS, Math.max(0, feedItemPos));
         fragment.setArguments(args);
         return fragment;
     }