s3:registry: Fix buffer truncation issues issues with gcc8

s3:registry: Fix buffer truncation issues issues with gcc8

../source3/registry/reg_perfcount.c: In function ‘reg_perfcount_get_hkpd’:
../source3/registry/reg_perfcount.c:337:29: error: ‘snprintf’ output may be truncated before the last format character [-Werror=format-truncation=]
   snprintf(buf, buflen,"%d%s", key_part1, key_part2);

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13437

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>
(cherry picked from commit 29f6842ee86b768f3677b38c5640655e312c398e)