gpg: Fix use of uninit.value in listing sig subpkts.

* g10/parse-packet.c (dump_sig_subpkt): Print regex subpacket
sanitized.

We may not use "%s" to print an arbitrary buffer. At least "%.*s"
should have been used. However, it is in general preferable to escape
control characters while printf user data.

• Reported-by: Hanno Böck
• Signed-off-by: Werner Koch <wk@gnupg.org>