Call unsafeResize in FlatMapReaders (facebookincubator#8129)

Summary:
X-link: facebookincubator/nimble#15

Pull Request resolved: facebookincubator#8129

We need to call unsafeResize in the DWRF FlatMapReader.

Reviewed By: zzhao0

Differential Revision: D52339055

fbshipit-source-id: 5ce8b38cebab5da7623c8a4923dc816cd94e5e12

velox/dwio/dwrf/reader/FlatMapColumnReader.cpp

@@ -686,7 +686,7 @@ void FlatMapStructEncodingColumnReader<T>::next(
   if (rowVector) {
     // Track children vectors in a local variable because readNulls may reset
     // the parent vector.
-    result->resize(numValues, false);
+    rowVector->unsafeResize(numValues, false);
     children = rowVector->children();
     DWIO_ENSURE_EQ(children.size(), keyNodes_.size());
   }