add [[cms::thread_safe]] to Registry::s_reg
edm::refToPtr now handles null Refs

Previous, edm::refToPtr would cause an exception to happen if it was passed a ‘null’ edm::Ref. This change now just returns a null edm::Ptr in such a case.
Merge pull request #2049 from Dr15Jones/dealWithNullRefsInConversionToPtr

Framework fixes -- edm::refToPtr now handles null Refs
Merge pull request #2048 from gartung/addattr-FWcore-PSET-sreg

Multicore fixes -- Add [[cms::thread_safe]] to non-const function static var Registry::instance()::s_reg