Fix occasional crash on exit because of statis network access manager

see https://bugreports.qt-project.org/browse/QTBUG-25789

src/core/qgsapplication.cpp

@@ -17,6 +17,7 @@
 #include "qgslogger.h"
 #include "qgsmaplayerregistry.h"
 #include "qgsproviderregistry.h"
+#include "qgsnetworkaccessmanager.h"
 #include "qgsexception.h"
 #include "qgsgeometry.h"
 
@@ -609,6 +610,8 @@ void QgsApplication::exitQgis()
   delete QgsMapLayerRegistry::instance();
 
   delete QgsProviderRegistry::instance();
+
+  delete QgsNetworkAccessManager::instance();
 }
 
 QString QgsApplication::showSettings()

src/core/qgsnetworkaccessmanager.cpp

@@ -86,10 +86,17 @@ class QgsNetworkProxyFactory : public QNetworkProxyFactory
     }
 };
 
-QgsNetworkAccessManager *QgsNetworkAccessManager::instance()
+//
+// Static calls to enforce singleton behaviour
+//
+QgsNetworkAccessManager* QgsNetworkAccessManager::sInstance = 0;
+QgsNetworkAccessManager* QgsNetworkAccessManager::instance()
 {
-  static QgsNetworkAccessManager sInstance;
-  return &sInstance;
+  if ( sInstance == 0 )
+  {
+    sInstance = new QgsNetworkAccessManager();
+  }
+  return sInstance;
 }
 
 QgsNetworkAccessManager::QgsNetworkAccessManager( QObject *parent )

src/core/qgsnetworkaccessmanager.h

@@ -48,7 +48,7 @@ class CORE_EXPORT QgsNetworkAccessManager : public QNetworkAccessManager
   public:
     //! returns a pointer to the single instance
     // and creates that instance on the first call.
-    static QgsNetworkAccessManager *instance();
+    static QgsNetworkAccessManager* instance();
 
     QgsNetworkAccessManager( QObject *parent = 0 );
 
@@ -100,6 +100,7 @@ class CORE_EXPORT QgsNetworkAccessManager : public QNetworkAccessManager
     QNetworkProxy mFallbackProxy;
     QStringList mExcludedURLs;
     bool mUseSystemProxy;
+    static QgsNetworkAccessManager* sInstance;
 };
 
 #endif // QGSNETWORKACCESSMANAGER_H

tests/src/core/testqgsnetworkcontentfetcher.cpp

@@ -16,6 +16,7 @@
  ***************************************************************************/
 
 #include "qgsnetworkcontentfetcher.h"
+#include "qgsapplication.h"
 #include <QObject>
 #include <QtTest>
 #include <QNetworkReply>
@@ -44,22 +45,21 @@ class TestQgsNetworkContentFetcher: public QObject
 
 void TestQgsNetworkContentFetcher::initTestCase()
 {
-
+  QgsApplication::init();
+  QgsApplication::initQgis();
 }
 
 void TestQgsNetworkContentFetcher::cleanupTestCase()
 {
-
+  QgsApplication::exitQgis();
 }
 
 void TestQgsNetworkContentFetcher::init()
 {
-
 }
 
 void TestQgsNetworkContentFetcher::cleanup()
 {
-
 }
 
 void TestQgsNetworkContentFetcher::fetchEmptyUrl()