core: Add commented debug helper to posix-io.c.
core: Add commented debug helper to posix-io.c.

* src/posix-io.c (_gpgme_is_fd_valid): New out-commented function.
--

This code is sometimes useful to track down invalid file descriptors.
We keep it commented in the code.

Signed-off-by: Werner Koch <13bd1fc5137f503b0a8355ad400e7a87ebb45b25@gnupg.org>

gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme