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.
Now, with a good variable name as above, you don't need this comment anymore. Comments are good, but self-explaining code is better ;)