ARROW-14305: [C++][Compute] Fixing Valgrind errors in hash join node tests
ARROW-14305: [C++][Compute] Fixing Valgrind errors in hash join node tests

On verifying contents of the Boolean column output from hash join test, Valgrind reported uninitialized data access.
I added clearing of an output bit vector for Boolean column before decoding.
This seems to have fixed Valgrind errors.

Closes #11395 from michalursa/ARROW-14305-hash-join-valgrind

Authored-by: michalursa <a2df1e659c9fd2578de0a26565357cb273292eeb@ursacomputing.com>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

icexelloss/arrow,apache/arrow,apache/arrow,wesm/arrow,laurentgo/arrow,icexelloss/arrow,apache/arrow,icexelloss/arrow,icexelloss/arrow,laurentgo/arrow,apache/arrow,wesm/arrow,laurentgo/arrow,kou/arrow,kou/arrow,laurentgo/arrow,icexelloss/arrow,kou/arrow,laurentgo/arrow,kou/arrow,wesm/arrow,laurentgo/arrow,wesm/arrow,apache/arrow,kou/arrow,kou/arrow,wesm/arrow,wesm/arrow,icexelloss/arrow,icexelloss/arrow,apache/arrow,kou/arrow,kou/arrow,laurentgo/arrow,laurentgo/arrow,icexelloss/arrow,icexelloss/arrow,kou/arrow,laurentgo/arrow,laurentgo/arrow,apache/arrow,apache/arrow,wesm/arrow,wesm/arrow,laurentgo/arrow,apache/arrow,apache/arrow,wesm/arrow,apache/arrow,laurentgo/arrow,icexelloss/arrow,wesm/arrow,kou/arrow,apache/arrow,laurentgo/arrow,icexelloss/arrow,wesm/arrow,apache/arrow,icexelloss/arrow,kou/arrow,kou/arrow,laurentgo/arrow,kou/arrow,wesm/arrow,kou/arrow,wesm/arrow,icexelloss/arrow,apache/arrow,icexelloss/arrow,wesm/arrow,laurentgo/arrow