Merge palloc()/MemSet(0) calls into a single palloc0() call.

contrib/dblink/dblink.c

contrib/intarray/_int.c

src/backend/access/common/indextuple.c

src/backend/access/common/tupdesc.c

src/backend/access/gist/gist.c

src/backend/access/nbtree/nbtsort.c

src/backend/access/rtree/rtproc.c

src/backend/catalog/index.c

src/backend/commands/analyze.c

src/backend/commands/copy.c

src/backend/commands/explain.c

src/backend/commands/opclasscmds.c

src/backend/commands/prepare.c

src/backend/commands/trigger.c

src/backend/commands/vacuumlazy.c

src/backend/executor/execMain.c

src/backend/executor/functions.c

src/backend/executor/nodeAgg.c

src/backend/executor/nodeAppend.c

src/backend/executor/nodeSort.c