QbsProjectManager: Fix null pointer accesses.

Change-Id: Ie57ecc36a676a3744317170eed7c4265becefe1b
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/qbsprojectmanager/qbsbuildstep.cpp

@@ -76,8 +76,10 @@ QbsBuildStep::QbsBuildStep(ProjectExplorer::BuildStepList *bsl, const QbsBuildSt
 QbsBuildStep::~QbsBuildStep()
 {
     cancel();
-    m_job->deleteLater();
-    m_job = 0;
+    if (m_job) {
+        m_job->deleteLater();
+        m_job = 0;
+    }
     delete m_parser;
 }
 

src/plugins/qbsprojectmanager/qbscleanstep.cpp

@@ -72,8 +72,10 @@ QbsCleanStep::QbsCleanStep(ProjectExplorer::BuildStepList *bsl, const QbsCleanSt
 QbsCleanStep::~QbsCleanStep()
 {
     cancel();
-    m_job->deleteLater();
-    m_job = 0;
+    if (m_job) {
+        m_job->deleteLater();
+        m_job = 0;
+    }
 }
 
 bool QbsCleanStep::init()