g10: Fix undefined behavior when EOF in parsing packet for S2K.

* g10/parse-packet.c (parse_symkeyenc): Use iobuf_get_noeof.
(parse_key): Likewise.

When EOF comes at parsing s2k.count, it is possible the value will
be (unsigned long)-1. Then, the result of S2K_DECODE_COUNT will be
undefined. This patch fixes undefined behavior.

• Reported-by: Philippe Antoine
• GnuPG-bug-id: T4093
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>