rxgen: avoid an unimportant memory leak in docppline()

rxgen: avoid an unimportant memory leak in docppline()

If cpp emitted
  # 1234 ""
docppline() would leak a small buffer.  Just free it.
The static analyzer will thank you.

Change-Id: I34ae6e228469a900139375f2a4758855922cf0ef
Reviewed-on: http://gerrit.openafs.org/7871
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>