Fix incorrect loop counts in tidbitmap.c.

A couple of places that should have been iterating over WORDS_PER_CHUNK
words were iterating over WORDS_PER_PAGE words instead.  This thinko
accidentally failed to fail, because (at least on common architectures
with default BLCKSZ) WORDS_PER_CHUNK is a bit less than WORDS_PER_PAGE,
and the extra words being looked at were always zero so nothing happened.
Still, it's a bug waiting to happen if anybody ever fools with the
parameters affecting TIDBitmap sizes, and it's a small waste of cycles
too.  So back-patch to all active branches.

Etsuro Fujita

Author: tglsfdc

src/backend/nodes/tidbitmap.c

@@ -361,7 +361,7 @@ tbm_union_page(TIDBitmap *a, const PagetableEntry *bpage)
 	if (bpage->ischunk)
 	{
 		/* Scan b's chunk, mark each indicated page lossy in a */
-		for (wordnum = 0; wordnum < WORDS_PER_PAGE; wordnum++)
+		for (wordnum = 0; wordnum < WORDS_PER_CHUNK; wordnum++)
 		{
 			bitmapword	w = bpage->words[wordnum];
 
@@ -473,7 +473,7 @@ tbm_intersect_page(TIDBitmap *a, PagetableEntry *apage, const TIDBitmap *b)
 		/* Scan each bit in chunk, try to clear */
 		bool		candelete = true;
 
-		for (wordnum = 0; wordnum < WORDS_PER_PAGE; wordnum++)
+		for (wordnum = 0; wordnum < WORDS_PER_CHUNK; wordnum++)
 		{
 			bitmapword	w = apage->words[wordnum];