Fix broken logic in lazy_vacuum_heap.

As noted by Tom Lane, the previous coding in this area, which I
introduced in commit bbb6e559c4, was
poorly tested and caused the vacuum's second heap to go into what would
have been an infinite loop but for the fact that it eventually caused a
memory allocation failure.  This version seems to work better.

src/backend/commands/vacuumlazy.c

@@ -991,7 +991,11 @@ lazy_vacuum_heap(Relation onerel, LVRelStats *vacrelstats)
 		buf = ReadBufferExtended(onerel, MAIN_FORKNUM, tblk, RBM_NORMAL,
 								 vac_strategy);
 		if (!ConditionalLockBufferForCleanup(buf))
+		{
+			ReleaseBuffer(buf);
+			++tupindex;
 			continue;
+		}
 		tupindex = lazy_vacuum_page(onerel, tblk, buf, tupindex, vacrelstats);
 
 		/* Now that we've compacted the page, record its available space */