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

• GnuPG-bug-id: T4012