Reduce scope of heap vacuum per_buffer_data

Move lazy_scan_heap()'s per_buffer_data variable into a tighter scope.
In lazy_scan_heap()'s phase I heap vacuuming, the read stream API
returns a pointer to the next block number to vacuum. As long as
read_stream_next_buffer() returns a valid buffer, per_buffer_data should
always be valid.

Move per_buffer_data into a tighter scope and make sure it is reset to
NULL on each iteration so that we get a core dump instead of bogus data
from a previous block if something goes wrong in the read stream API.

Suggested-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/626104.1739729538%40sss.pgh.pa.us

src/backend/access/heap/vacuumlazy.c

@@ -1192,7 +1192,6 @@ lazy_scan_heap(LVRelState *vacrel)
 	BlockNumber rel_pages = vacrel->rel_pages,
 				blkno = 0,
 				next_fsm_block_to_vacuum = 0;
-	void	   *per_buffer_data = NULL;
 	BlockNumber orig_eager_scan_success_limit =
 		vacrel->eager_scan_remaining_successes; /* for logging */
 	Buffer		vmbuffer = InvalidBuffer;
@@ -1231,6 +1230,7 @@ lazy_scan_heap(LVRelState *vacrel)
 		Page		page;
 		uint8		blk_info = 0;
 		bool		has_lpdead_items;
+		void	   *per_buffer_data = NULL;
 		bool		vm_page_frozen = false;
 		bool		got_cleanup_lock = false;