Fix valgrind's "unaddressable bytes" whining about BRIN code.

brin_form_tuple calculated an exact tuple size, then palloc'd and
filled just that much.  Later, brin_doinsert or brin_doupdate would
MAXALIGN the tuple size and tell PageAddItem that that was the size
of the tuple to insert.  If the original tuple size wasn't a multiple
of MAXALIGN, the net result would be that PageAddItem would memcpy
a few more bytes than the palloc request had been for.

AFAICS, this is totally harmless in the real world: the error is a
read overrun not a write overrun, and palloc would certainly have
rounded the request up to a MAXALIGN multiple internally, so there's
no chance of the memcpy fetching off the end of memory.  Valgrind,
however, is picky to the byte level not the MAXALIGN level.

Fix it by pushing the MAXALIGN step back to brin_form_tuple.  (The other
possible source of tuples in this code, brin_form_placeholder_tuple,
was already producing a MAXALIGN'd result.)

In passing, be a bit more paranoid about internal allocations in
brin_form_tuple.

src/backend/access/brin/brin_pageops.c

@@ -55,7 +55,7 @@ brin_doupdate(Relation idxrel, BlockNumber pagesPerRange,
 	Buffer		newbuf;
 	bool		extended = false;
 
-	newsz = MAXALIGN(newsz);
+	Assert(newsz == MAXALIGN(newsz));
 
 	/* make sure the revmap is long enough to contain the entry we need */
 	brinRevmapExtend(revmap, heapBlk);
@@ -273,7 +273,7 @@ brin_doinsert(Relation idxrel, BlockNumber pagesPerRange,
 	ItemPointerData tid;
 	bool		extended = false;
 
-	itemsz = MAXALIGN(itemsz);
+	Assert(itemsz == MAXALIGN(itemsz));
 
 	/* Make sure the revmap is long enough to contain the entry we need */
 	brinRevmapExtend(revmap, heapBlk);

src/backend/access/brin/brin_tuple.c

@@ -103,9 +103,10 @@ brin_form_tuple(BrinDesc *brdesc, BlockNumber blkno, BrinMemTuple *tuple,
 
 	Assert(brdesc->bd_totalstored > 0);
 
-	values = palloc(sizeof(Datum) * brdesc->bd_totalstored);
-	nulls = palloc0(sizeof(bool) * brdesc->bd_totalstored);
-	phony_nullbitmap = palloc(sizeof(bits8) * BITMAPLEN(brdesc->bd_totalstored));
+	values = (Datum *) palloc(sizeof(Datum) * brdesc->bd_totalstored);
+	nulls = (bool *) palloc0(sizeof(bool) * brdesc->bd_totalstored);
+	phony_nullbitmap = (bits8 *)
+		palloc(sizeof(bits8) * BITMAPLEN(brdesc->bd_totalstored));
 
 	/*
 	 * Set up the values/nulls arrays for heap_fill_tuple
@@ -144,6 +145,9 @@ brin_form_tuple(BrinDesc *brdesc, BlockNumber blkno, BrinMemTuple *tuple,
 			values[idxattno++] = tuple->bt_columns[keyno].bv_values[datumno];
 	}
 
+	/* Assert we did not overrun temp arrays */
+	Assert(idxattno <= brdesc->bd_totalstored);
+
 	/* compute total space needed */
 	len = SizeOfBrinTuple;
 	if (anynulls)
@@ -160,12 +164,15 @@ brin_form_tuple(BrinDesc *brdesc, BlockNumber blkno, BrinMemTuple *tuple,
 
 	data_len = heap_compute_data_size(brtuple_disk_tupdesc(brdesc),
 									  values, nulls);
-
 	len += data_len;
 
+	len = MAXALIGN(len);
+
 	rettuple = palloc0(len);
 	rettuple->bt_blkno = blkno;
 	rettuple->bt_info = hoff;
+
+	/* Assert that hoff fits in the space available */
 	Assert((rettuple->bt_info & BRIN_OFFSET_MASK) == hoff);
 
 	/*