CDRIVER-2214 invalid free if aggregate fails
CDRIVER-2214 invalid free if aggregate fails

If aggregate is called with readConcern and the server is too old to
support it, aggregate returns early. Make sure the reply BSON is always
initialized, even with an early return.

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