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ARROW-6572: [C++] Fix Parquet decoding returning uninitialized data

This happens when reading RLE-encoded dict-encoded data with nulls.
Make sure we zero-initialize the data array slots corresponding to null entries.

Closes #5392 from pitrou/ARROW-6572-parquet-uninitialized-data and squashes the following commits:

42d91cd9c <Antoine Pitrou> ARROW-6572:  Fix Parquet decoding returning uninitialized data

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>