more reference fixes in descriptor.c
more reference fixes in descriptor.c

teoliphant/numpy-refactor,jasonmccampbell/numpy-refactor-sprint,jasonmccampbell/numpy-refactor-sprint,teoliphant/numpy-refactor,teoliphant/numpy-refactor,jasonmccampbell/numpy-refactor-sprint,teoliphant/numpy-refactor,teoliphant/numpy-refactor,jasonmccampbell/numpy-refactor-sprint