arrow

ARROW-5629: [C++] Fix Coverity issues

Nothing serious, just small cleanups.

Note I didn't enable all components when running the analysis, since it's quite long already. I left CUDA, Gandiva, Orc and Plasma disabled.

Author: Antoine Pitrou <antoine@python.org>

Closes #4595 from pitrou/ARROW-5629-fix-coverity-issues and squashes the following commits:

fa235f534 <Antoine Pitrou> ARROW-5629:  Fix Coverity issues