QbsProjectManager: Fix crash when unloading a building project

Note that unloading a building project is generally broken in that the
build manager waits indefinitely for the build step's finished() signal,
but that's a general problem which needs to get addressed in a different
patch. This one fixes a qbs-specific crash.

Fixes: QTCREATORBUG-23851
Change-Id: I93e254acd1c73fe7495d01fcad04ea9301130dd0
Reviewed-by: Ivan Komissarov <ABBAPOH@gmail.com>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/qbsprojectmanager/qbscleanstep.cpp

@@ -85,6 +85,15 @@ QbsCleanStep::~QbsCleanStep()
         m_session->disconnect(this);
 }
 
+void QbsCleanStep::dropSession()
+{
+    if (m_session) {
+        doCancel();
+        m_session->disconnect(this);
+        m_session = nullptr;
+    }
+}
+
 bool QbsCleanStep::init()
 {
     if (buildSystem()->isParsing() || m_session)