Fix variable initialization when using buffering build with GiST

This can cause valgrind to complain, as the flag marking a buffer as a
temporary copy was not getting initialized.

While on it, fill in with zeros newly-created buffer pages.  This does
not matter when loading a block from a temporary file, but it makes the
push of an index tuple into a new buffer page safer.

This has been introduced by 1d27dcf5, so backpatch all the way down to
9.4.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/15899-0d24fb273b3dd90c@postgresql.org
Backpatch-through: 9.4

src/backend/access/gist/gistbuildbuffers.c

@@ -138,6 +138,7 @@ gistGetNodeBuffer(GISTBuildBuffers *gfbb, GISTSTATE *giststate,
 		nodeBuffer->pageBlocknum = InvalidBlockNumber;
 		nodeBuffer->pageBuffer = NULL;
 		nodeBuffer->queuedForEmptying = false;
+		nodeBuffer->isTemp = false;
 		nodeBuffer->level = level;
 
 		/*
@@ -186,7 +187,7 @@ gistAllocateNewPageBuffer(GISTBuildBuffers *gfbb)
 {
 	GISTNodeBufferPage *pageBuffer;
 
-	pageBuffer = (GISTNodeBufferPage *) MemoryContextAlloc(gfbb->context,
+	pageBuffer = (GISTNodeBufferPage *) MemoryContextAllocZero(gfbb->context,
 															   BLCKSZ);
 	pageBuffer->prev = InvalidBlockNumber;