gpg: Sanitize diagnostic with the original file name.

* g10/mainproc.c (proc_plaintext): Sanitize verbose output.

This fixes a forgotten sanitation of user supplied data in a verbose
mode diagnostic. The mention CVE is about using this to inject
status-fd lines into the stderr output. Other harm good as well be
done. Note that GPGME based applications are not affected because
GPGME does not fold status output into stderr.

CVE-id: CVE-2018-12020
(cherry picked from commit 13f135c7a252cc46cff96e75968d92b6dc8dce1b)

• GnuPG-bug-id: T4012