Fix crash at exit when there is a log after the Logger has been destroyed

Use a proper static Logger instead of allocating one,  and cleanup
the QTMessageLogger when it is destroyed

src/mirall/logger.cpp

@@ -27,6 +27,9 @@ static void mirallLogCatcher(QtMsgType type, const char *msg)
   // qDebug() exports to local8Bit, which is not always UTF-8
   Logger::instance()->mirallLog( QString::fromLocal8Bit(msg) );
 }
+static void qInstallMessageHandler(QtMsgHandler h) {
+    qInstallMsgHandler(h);
+}
 #else
 static void mirallLogCatcher(QtMsgType, const QMessageLogContext &ctx, const QString &message) {
     Q_UNUSED(ctx);
@@ -38,35 +41,23 @@ static void mirallLogCatcher(QtMsgType, const QMessageLogContext &ctx, const QSt
 }
 #endif
 
-Logger* Logger::_instance=0;
-
-Logger::Logger( QObject* parent)
-: QObject(parent),
-  _showTime(true), _doLogging(false), _doFileFlush(false), _logExpire(0)
-{
-}
-
 Logger *Logger::instance()
 {
-    if( !Logger::_instance ) {
+    static Logger log;
-        Logger::_instance = new Logger;
+    return &log;
-#if QT_VERSION < QT_VERSION_CHECK(5, 0, 0)
-        qInstallMsgHandler( mirallLogCatcher );
-#else
-        qInstallMessageHandler(mirallLogCatcher);
-#endif
-    }
-    return Logger::_instance;
 }
 
-void Logger::destroy()
+Logger::Logger( QObject* parent) : QObject(parent),
+  _showTime(true), _doLogging(false), _doFileFlush(false), _logExpire(0)
 {
-    if( Logger::_instance ) {
+    qInstallMessageHandler(mirallLogCatcher);
-        delete Logger::_instance;
-        Logger::_instance = 0;
-    }
 }
 
+Logger::~Logger() {
+    qInstallMessageHandler(0);
+}
+
+
 void Logger::postGuiLog(const QString &title, const QString &message)
 {
     emit guiLog(title, message);

src/mirall/logger.h

@@ -41,6 +41,7 @@ class OWNCLOUDSYNC_EXPORT Logger : public QObject
 {
   Q_OBJECT
 public:
+
   void log(Log log);
 
   static void csyncLog( const QString& message );
@@ -49,7 +50,6 @@ public:
   const QList<Log>& logs() const {return _logs;}
 
   static Logger* instance();
-  static void destroy();
 
   void postGuiLog(const QString& title, const QString& message);
   void postOptionalGuiLog(const QString& title, const QString& message);
@@ -69,14 +69,12 @@ signals:
 public slots:
   void enterNextLogFile();
 
-protected:
+private:
   Logger(QObject* parent=0);
+  ~Logger();
   QList<Log> _logs;
   bool       _showTime;
   bool       _doLogging;
-
-  static Logger* _instance;
-
   QFile       _logFile;
   bool        _doFileFlush;
   int         _logExpire;