List initialize blobStream_ in StringDictionaryColumnReader (#6855)

Summary:
Pull Request resolved: #6855

Below diffs allow us to add more private members to initialization list. Yay!

Reviewed By: Magoja

Differential Revision: D49840659

fbshipit-source-id: 42a33d5b55d3be27c33948ced14c9dc79770712c

velox/dwio/dwrf/reader/ColumnReader.cpp

@@ -1149,6 +1149,10 @@ StringDictionaryColumnReader::StringDictionaryColumnReader(
       dictionaryCount_(stripe.getEncoding(encodingKey).dictionarysize()),
       lastStrideIndex_(-1),
       provider_(stripe.getStrideIndexProvider()),
+      blobStream_(stripe.getStream(
+          encodingKey.forKind(proto::Stream_Kind_DICTIONARY_DATA),
+          streamLabels.label(),
+          false)),
       returnFlatVector_(stripe.getRowReaderOptions().getReturnFlatVector()) {
   dictIndex_ = makeRleDecoder(
       encodingKey,
@@ -1168,11 +1172,6 @@ StringDictionaryColumnReader::StringDictionaryColumnReader(
       proto::Stream_Kind_LENGTH,
       memoryPool_);
 
-  blobStream_ = stripe.getStream(
-      encodingKey.forKind(proto::Stream_Kind_DICTIONARY_DATA),
-      streamLabels.label(),
-      false);
-
   // handle in dictionary stream
   std::unique_ptr<dwio::common::SeekableInputStream> inDictStream =
       stripe.getStream(