util: storage: Fix possible crash when source path is NULL
util: storage: Fix possible crash when source path is NULL

Some storage protocols allow to have the @path field in struct
virStorageSource set to NULL. Add NULLSTR() wrappers to handle this
possibility until I finish the storage source error formatter.

VenkatDatta/libvirt,datto/libvirt,andreabolognani/libvirt,jardasgit/libvirt,agx/libvirt,crobinso/libvirt,datto/libvirt,datto/libvirt,crobinso/libvirt,agx/libvirt,olafhering/libvirt,crobinso/libvirt,jardasgit/libvirt,taget/libvirt,nertpinx/libvirt,zippy2/libvirt,agx/libvirt,rlaager/libvirt,zippy2/libvirt,eskultety/libvirt,andreabolognani/libvirt,jfehlig/libvirt,jfehlig/libvirt,libvirt/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,olafhering/libvirt,datto/libvirt,rlaager/libvirt,jardasgit/libvirt,olafhering/libvirt,zippy2/libvirt,jardasgit/libvirt,rlaager/libvirt,taget/libvirt,eskultety/libvirt,andreabolognani/libvirt,zippy2/libvirt,rlaager/libvirt,datto/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,taget/libvirt,nertpinx/libvirt,eskultety/libvirt,taget/libvirt,libvirt/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,libvirt/libvirt,libvirt/libvirt,crobinso/libvirt,nertpinx/libvirt,jfehlig/libvirt,andreabolognani/libvirt,nertpinx/libvirt,olafhering/libvirt,agx/libvirt,agx/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,rlaager/libvirt,taget/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,eskultety/libvirt,eskultety/libvirt