Don't crash on empty appcasts.

This bug was introduced by PR #14 in ec85b3c.

Fixes #24.

src/appcast.cpp

@@ -187,7 +187,7 @@ bool is_windows_item(const Appcast &item)
                                Appcast class
  *--------------------------------------------------------------------------*/
 
-void Appcast::Load(const std::string& xml)
+Appcast Appcast::Load(const std::string& xml)
 {
     XML_Parser p = XML_ParserCreateNS(NULL, NS_SEP);
     if ( !p )
@@ -211,12 +211,16 @@ void Appcast::Load(const std::string& xml)
 
     XML_ParserFree(p);
 
+    if (ctxt.items.empty())
+        return Appcast(); // invalid
+
     // Search for first <item> which has "sparkle:os=windows" attribute.
     // If none, use the first item.
     std::vector<Appcast>::iterator it = std::find_if(ctxt.items.begin(), ctxt.items.end(), is_windows_item);
-    if (it == ctxt.items.end())
-        it = ctxt.items.begin();
-    *this = *it;
+    if (it != ctxt.items.end())
+        return *it;
+    else
+        return ctxt.items.front();
 }
 
 } // namespace winsparkle

src/appcast.h

@@ -52,20 +52,25 @@ struct Appcast
     /// Description of the update
     std::string Description;
 
-	// Operating system
-	std::string Os;
+    // Operating system
+    std::string Os;
 
     /**
         Initializes the struct with data from XML appcast feed.
 
         If the feed contains multiple entries, only the latest one is read,
-        the rest is ignored.
+        the rest is ignored. Entries that are not appliable (e.g. for different
+        OS) are likewise skipped.
 
         Throws on error.
+        Returns NULL if no error ocurred, but there was no update in the appcast.
 
         @param xml Appcast feed data.
      */
-    void Load(const std::string& xml);
+    static Appcast Load(const std::string& xml);
+
+    /// Returns true if the struct constains valid data.
+    bool IsValid() const { return !Version.empty(); }
 };
 
 } // namespace winsparkle

src/updatechecker.cpp

@@ -232,16 +232,15 @@ void UpdateChecker::Run()
         StringDownloadSink appcast_xml;
         DownloadFile(url, &appcast_xml, GetAppcastDownloadFlags());
 
-        Appcast appcast;
-        appcast.Load(appcast_xml.data);
+        Appcast appcast = Appcast::Load(appcast_xml.data);
 
         Settings::WriteConfigValue("LastCheckTime", time(NULL));
 
         const std::string currentVersion =
                 WideToAnsi(Settings::GetAppBuildVersion());
 
         // Check if our version is out of date.
-        if ( CompareVersions(currentVersion, appcast.Version) >= 0 )
+        if ( !appcast.IsValid() || CompareVersions(currentVersion, appcast.Version) >= 0 )
         {
             // The same or newer version is already installed.
             UI::NotifyNoUpdates();