arrow

ARROW-2462: [C++] Fix Segfault in UnpackBinaryDictionary

Discovered this through using pyarrow and dealing with RecordBatch Streams and parquet. The issue can be replicated as follows:

```python
import pyarrow as pa
import pyarrow.parquet as pq

# create record batch with 1 dictionary column
indices = pa.array([1,0,1,1,0])
dictionary = pa.array(['Foo', 'Bar'])
dict_array = pa.DictionaryArray.from_arrays(indices, dictionary)
rb = pa.RecordBatch.from_arrays( [ dict_array ], [ 'd0' ] )

# write out using RecordBatchStreamWriter
sink = pa.BufferOutputStream()
writer = pa.RecordBatchStreamWriter(sink, rb.schema)
writer.write_batch(rb)
writer.close()
buf = sink.get_result()

# read in and try to write parquet table
reader = pa.open_stream(buf)
tbl = reader.read_all()
pq.write_table(tbl, 'dict_table.parquet') # SEGFAULTS
```

When writing record batch streams, if there are no nulls in an array, Arrow will put a placeholder nullptr instead of putting the full bitmap of 1s, when deserializing that stream, the bitmap for the nulls isn't populated and is left to being a nullptr. When attempting to write this table via pyarrow.parquet, you end up [here](https://github.com/apache/parquet-cpp/blob/master/src/parquet/arrow/writer.cc#L963) in the parquet writer code which attempts to Cast the dictionary to a non-dictionary representation. Since the null count isn't checked before creating a BitmapReader, the BitmapReader is constructed with a nullptr for the bitmap_data, but a non-zero length which then segfaults in the constructor [here](https://github.com/apache/arrow/blob/master/cpp/src/arrow/util/bit-util.h#L415) because `bitmap` is null.

So a simple check of the null count before constructing the BitmapReader avoids the segfault.

Author: Matt Topol <zotthewizard@gmail.com>
Author: Matthew Topol <mtopol@factset.com>

Closes #1896 from zeroshade/fix_cast and squashes the following commits:

631b95c <Matthew Topol> Adding check in the unit test to validate the data itself too
e085476 <Matt Topol> Add unit test for unpacking
8b86540 <Matt Topol> ARROW-2462:  Fix Segfault in UnpackBinaryDictionary / UnpackFixedSizeBinaryDictionary