Fix system information updating during hyperjump

I experienced following crash, when tried to see star system information
during hyperjump:

Started hyperspacing...
pioneer: SmartPtr.h:28: T* SmartPtrBase<Derived, T>::operator->() const
[with Derived = RefCountedPtr<StarSystem>; T = StarSystem]: Assertion
`m_ptr' failed.
Aborted (core dumped)

This commit adds more accurate checks for case when current system is
not defined.