Fix array overflow in AO concurrency handling

In the function ao_foreach_extent_file() an array concurrency is used to
store the concurrent levels, its size was AOTupleId_MaxSegmentFileNum,
which equals to (MAX_AOREL_CONCURRENCY - 1).  However the possible index
range is [0, MAX_AOREL_CONCURRENCY - 1], it exceeds the original size of
the array, so an array overflow can happen at runtime.

Fixed by correcting the array size, also define the size with
MAX_AOREL_CONCURRENCY instead of AOTupleId_MaxSegmentFileNum.
Reviewed-by: NZhenghua Lyu <zlv@pivotal.io>
Reviewed-by: NNing Yu <nyu@pivotal.io>

src/backend/access/appendonly/aomd_filehandler.c

@@ -68,7 +68,7 @@ ao_foreach_extent_file(ao_extent_callback callback, void *ctx)
 {
     int segno;
     int colnum;
-    int concurrency[AOTupleId_MaxSegmentFileNum];
+    int concurrency[MAX_AOREL_CONCURRENCY];
     int concurrencySize;
 
     /*