Fix overriding the default session

To reproduce the bug:
1) Load a project into the default session.cpp
2) Close creator (which saves the session)
3) Start Creator
4) Create a new session and switch to it => This calls createImpl
   and overrides the default session, even though it shouldn't
5) Load the default session, which will now be empty

Change-Id: I7c23fba8c1f9fd32cc95b3b9206afc422c1f0133
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>

src/plugins/projectexplorer/session.cpp

@@ -545,7 +545,9 @@ bool SessionManager::createImpl(const QString &fileName)
     bool success = true;
 
     if (!m_file->fileName().isEmpty()) {
-        if (!save() || !clear())
+        if (isDefaultVirgin()) {
+            // do not save initial and virgin default session
+        } else if (!save() || !clear())
             success = false;
     }