ARROW-8006: [C++] Initialize spaced data when reading nulls from Parquet

Closes #6544 from pitrou/ARROW-8006-parquet-uninitialized-data and squashes the following commits:

601eba2 <Wes McKinney> Add unit test that fails without the patch
b614686 <Antoine Pitrou> ARROW-8006:  Initialize spaced data when reading nulls from Parquet

Lead-authored-by: Antoine Pitrou <antoine@python.org>
Co-authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>

cpp/src/arrow/util/rle_encoding.h

@@ -333,6 +333,7 @@ inline int RleDecoder::GetBatchSpaced(int batch_size, int null_count,
   DCHECK_GE(bit_width_, 0);
   int values_read = 0;
   int remaining_nulls = null_count;
+  T zero = {};
 
   arrow::internal::BitmapReader bit_reader(valid_bits, valid_bits_offset, batch_size);
 
@@ -383,6 +384,7 @@ inline int RleDecoder::GetBatchSpaced(int batch_size, int null_count,
             *out = indices[literals_read];
             literals_read++;
           } else {
+            *out = zero;
             skipped++;
           }
           ++out;
@@ -393,6 +395,7 @@ inline int RleDecoder::GetBatchSpaced(int batch_size, int null_count,
         remaining_nulls -= skipped;
       }
     } else {
+      *out = zero;
       ++out;
       values_read++;
       remaining_nulls--;

cpp/src/parquet/encoding_test.cc

@@ -904,7 +904,21 @@ TEST_F(DictEncoding, CheckDecodeIndicesSpaced) {
       actual_num_values = dict_decoder_->DecodeIndicesSpaced(
           num_values_, null_count_, valid_bits_, 0, builder.get());
     }
-    CheckDict(actual_num_values, *builder);
+    ASSERT_EQ(actual_num_values, num_values_ - null_count_);
+    std::shared_ptr<arrow::Array> actual;
+    ASSERT_OK(builder->Finish(&actual));
+    ASSERT_ARRAYS_EQUAL(*actual, *expected_dict_);
+
+    // Check that null indices are zero-initialized
+    const auto& dict_actual = checked_cast<const arrow::DictionaryArray&>(*actual);
+    const auto& indices = checked_cast<const arrow::Int32Array&>(*dict_actual.indices());
+
+    auto raw_values = indices.raw_values();
+    for (int64_t i = 0; i < indices.length(); ++i) {
+      if (indices.IsNull(i) && raw_values[i] != 0) {
+        FAIL() << "Null slot not zero-initialized";
+      }
+    }
   }
 }