Bugfix: heap-buffer-overflow when querying array (StarRocks#921)

Close issue StarRocks#920

Cannot rely on `array_reader->is_nullable()` before `array_reader` been
initialized.

be/src/storage/rowset/segment_v2/column_reader.cpp

@@ -72,7 +72,7 @@ Status ColumnReader::create(MemTracker* mem_tracker, const ColumnReaderOptions&
         col++;
         array_reader->_sub_readers->emplace_back(std::move(element_reader));
 
-        if (array_reader->is_nullable()) {
+        if (meta->is_nullable()) {
             std::unique_ptr<ColumnReader> null_reader;
             RETURN_IF_ERROR(ColumnReader::create(mem_tracker, opts, meta->mutable_children_columns(col), file_name,
                                                  &null_reader));