GoodSeedProducer: initialize TMVA reader pointers to zero

If beginRun() is not called, TMVA pointers are garbage and will
cause SEGFAULT in dtor of GoodSeedProducer, once it tries to
deallocate them.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #4053 from davidlt/GoodSeedProducer-dtor-segfault

Good seed producer dtor segfault.