Added null check when closing network request

Fixes #2307

core/src/main/java/de/danoeh/antennapod/core/gpoddernet/GpodnetService.java

@@ -568,8 +568,10 @@ public class GpodnetService {
             e.printStackTrace();
             throw new GpodnetServiceException(e);
         } finally {
+            if (body != null) {
                 body.close();
             }
+        }
         return responseString;
     }