Qmake: Work around crash

The condition that's checked now triggered when playing around
with changing the JS expression that creating the default build
directory name.

Change-Id: I221be8cefb6918c10c383c23ee7cde73d3683e40
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/qmakeprojectmanager/qmakenodes.cpp

@@ -112,6 +112,7 @@ bool QmakeBuildSystem::supportsAction(Node *context, ProjectAction action, const
             }
             QTC_ASSERT(proFileNode, return false);
             pro = proFileNode->proFile();
+            QTC_ASSERT(pro, return false);
             t = pro->projectType();
         }