Check appcast item correctness

Check if an item is valid, i.e. has either a usable enclosure or a link
to open in the browser.

src/appcast.cpp

@@ -344,7 +344,7 @@ void XMLCALL OnEndElement(void *data, const char *name)
 				}
             }
 
-            if (is_compatible_with_windows_version(item))
+            if (item.IsValid() && is_compatible_with_windows_version(item))
             {
                 ctxt.all_items.push_back(item);
 

src/appcast.h

@@ -93,7 +93,7 @@ struct Appcast
     static Appcast Load(const std::string& xml);
 
     /// Returns true if the struct constains valid data.
-    bool IsValid() const { return !Version.empty(); }
+    bool IsValid() const { return !Version.empty() && (HasDownload() || !WebBrowserURL.empty()); }
 
     /// If true, then download and install the update ourselves.
     /// If false, launch a web browser to WebBrowserURL.