CDRIVER-2341 crash in mongoc_set_destroy
CDRIVER-2341 crash in mongoc_set_destroy

The set-item destructor should be optional. Don't call it if it's NULL.

ajdavis/mongo-c-driver,derickr/mongo-c-driver,acmorrow/mongo-c-driver,beingmeta/mongo-c-driver,mongodb/mongo-c-driver,remicollet/mongo-c-driver,rcsanchez97/mongo-c-driver,remicollet/mongo-c-driver,mongodb/mongo-c-driver,ajdavis/mongo-c-driver,acmorrow/mongo-c-driver,rcsanchez97/mongo-c-driver,derickr/mongo-c-driver,derickr/mongo-c-driver,remicollet/mongo-c-driver,acmorrow/mongo-c-driver,rcsanchez97/mongo-c-driver,derickr/mongo-c-driver,mongodb/mongo-c-driver,remicollet/mongo-c-driver,ajdavis/mongo-c-driver,beingmeta/mongo-c-driver,beingmeta/mongo-c-driver,jmikola/mongo-c-driver,remicollet/mongo-c-driver,jmikola/mongo-c-driver,rcsanchez97/mongo-c-driver,beingmeta/mongo-c-driver,derickr/mongo-c-driver,derickr/mongo-c-driver,mongodb/mongo-c-driver,beingmeta/mongo-c-driver,remicollet/mongo-c-driver,beingmeta/mongo-c-driver,ajdavis/mongo-c-driver,remicollet/mongo-c-driver,rcsanchez97/mongo-c-driver,jmikola/mongo-c-driver,mongodb/mongo-c-driver,acmorrow/mongo-c-driver,mongodb/mongo-c-driver,acmorrow/mongo-c-driver,beingmeta/mongo-c-driver,jmikola/mongo-c-driver,jmikola/mongo-c-driver,rcsanchez97/mongo-c-driver,acmorrow/mongo-c-driver,ajdavis/mongo-c-driver,ajdavis/mongo-c-driver,beingmeta/mongo-c-driver,ajdavis/mongo-c-driver,jmikola/mongo-c-driver,derickr/mongo-c-driver,mongodb/mongo-c-driver,rcsanchez97/mongo-c-driver,acmorrow/mongo-c-driver,jmikola/mongo-c-driver