remove redundant initializations

Reported-by: Ranier Vilela

Discussion: https://postgr.es/m/CAEudQAo1+AcGppxDSg8k+zF4+Kv+eJyqzEDdbpDg58-=MQcerQ@mail.gmail.com

Author: Ranier Vilela

Backpatch-through: master

src/backend/access/gist/gistxlog.c

@@ -405,7 +405,6 @@ gistRedoPageReuse(XLogReaderState *record)
 		 * logged value is very old, so that XID wrap-around already happened
 		 * on it, there can't be any snapshots that still see it.
 		 */
-		nextXid = ReadNextFullTransactionId();
 		diff = U64FromFullTransactionId(nextXid) -
 			U64FromFullTransactionId(latestRemovedFullXid);
 		if (diff < MaxTransactionId / 2)

src/backend/catalog/heap.c

@@ -3436,7 +3436,7 @@ List *
 heap_truncate_find_FKs(List *relationIds)
 {
 	List	   *result = NIL;
-	List	   *oids = list_copy(relationIds);
+	List	   *oids;
 	List	   *parent_cons;
 	ListCell   *cell;
 	ScanKeyData key;

src/backend/storage/smgr/md.c

@@ -734,9 +734,11 @@ mdwrite(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 BlockNumber
 mdnblocks(SMgrRelation reln, ForkNumber forknum)
 {
-	MdfdVec    *v = mdopenfork(reln, forknum, EXTENSION_FAIL);
+	MdfdVec    *v;
 	BlockNumber nblocks;
-	BlockNumber segno = 0;
+	BlockNumber segno;
+
+	mdopenfork(reln, forknum, EXTENSION_FAIL);
 
 	/* mdopen has opened the first segment */
 	Assert(reln->md_num_open_segs[forknum] > 0);

src/backend/utils/adt/json.c

@@ -990,7 +990,7 @@ catenate_stringinfo_string(StringInfo buffer, const char *addon)
 Datum
 json_build_object(PG_FUNCTION_ARGS)
 {
-	int			nargs = PG_NARGS();
+	int			nargs;
 	int			i;
 	const char *sep = "";
 	StringInfo	result;

src/backend/utils/mmgr/mcxt.c

@@ -476,7 +476,7 @@ MemoryContextMemAllocated(MemoryContext context, bool recurse)
 
 	if (recurse)
 	{
-		MemoryContext child = context->firstchild;
+		MemoryContext child;
 
 		for (child = context->firstchild;
 			 child != NULL;