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ARROW-4653: [C++] Fix bug in decimal multiply

- fixed a bug which was causing multiply to return an incorrect
  value when both args are -ve.

Author: Pindikura Ravindra <ravindra@dremio.com>

Closes #3728 from pravindra/multbug and squashes the following commits:

7a7b0c2e <Pindikura Ravindra> ARROW-4653: Fix bug in decimal multiply