comm: flush stdout for error checking prior to exiting

UNIX conformance wants utilities to catch any errors when doing I/O, as
opposed to relying on the implicit flush upon exit.
comm currently does not do that.
This commit adds handling of I/O errors on stdout prior to exit.

Reviewed by: imp, allanjude
Sponsored by: Apple Inc.
Differential Revision: https://reviews.freebsd.org/D45439