heap_update() must recheck tuple after unlocking and relocking buffer.

Bug found by Alvaro Herrera, fix suggested by Heikki Linnakangas
and reviewed by Tom Lane.

src/backend/access/heap/heapam.c

@@ -2645,13 +2645,16 @@ l2:
 	 * visible while we were busy locking the buffer, or during some subsequent
 	 * window during which we had it unlocked, we'll have to unlock and
 	 * re-lock, to avoid holding the buffer lock across an I/O.  That's a bit
-	 * unfortunate, but hopefully shouldn't happen often.
+	 * unfortunate, esepecially since we'll now have to recheck whether the
+	 * tuple has been locked or updated under us, but hopefully it won't
+	 * happen very often.
 	 */
 	if (vmbuffer == InvalidBuffer && PageIsAllVisible(page))
 	{
 		LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
 		visibilitymap_pin(relation, block, &vmbuffer);
 		LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
+		goto l2;
 	}
 
 	/*