comm: close a race condition when comm is fed from stdin

If one of the files has ended, we won't show the column, but we still
need to drain the file pointer to avoid potentially hitting a pipe
failure.

This commit moves the NULL offset checks inside show() so that getline()
and ferror() are still called on fp.

Reviewed by: allanjude
Sponsored by: Apple Inc.
Differential Revision: https://reviews.freebsd.org/D45440