Fix crash in item pager (#6542)

There should be no code path for feedItemPos to still be -1, but the
crash reports indicate that it does. So this is now the dirty fix to
avoid app crashes.

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

@@ -70,7 +70,7 @@ public class ItemPagerFragment extends Fragment implements MaterialToolbar.OnMen
         toolbar.setOnMenuItemClickListener(this);
 
         feedItems = getArguments().getLongArray(ARG_FEEDITEMS);
-        int feedItemPos = getArguments().getInt(ARG_FEEDITEM_POS);
+        final int feedItemPos = Math.max(0, getArguments().getInt(ARG_FEEDITEM_POS));
 
         pager = layout.findViewById(R.id.pager);
         // FragmentStatePagerAdapter documentation: