Ensure that also mostly unused SiSpacePointForSeed members are always initialised.
Some members which are only used when writing special seed ntuples are not always initialised. This caused occasional FPEs when the assignment operator was called. Now these members are always initialised to zero.
This fixes FPEs showing up in an athenaMT test using a clang10 build. This will likely also address ATLASRECTS-6141.