Fix brin index summarizing while vacuuming.
If the number of heap blocks is not multiples of pages per range, the
summarizing produces wrong summary information for the last brin index
tuple while vacuuming.
Problem reported by Tatsuo Ishii and fixed by Amit Langote.
Discussion at "[HACKERS] BRIN INDEX value (message id :
20150903.174935.
1946402199422994347.t-ishii@sraoss.co.jp)
Backpatched to 9.5 in which brin index was added.