Fix build, fix potential crash

[gregor-anich-uibk]

Sorry for breaking the old build system :disappointed:

The changes to PythonQt(Safe)ObjectPtr are only needed if one calls Py_Finalize() I believe. In this case the destructor calls Py_DECREF after the python interpreter was destroyed. For "normal" users this should never happen, but still shouldn't hurt to have this check and safe others time when debugging their code.