dmi data: fix failure of reading dmi data for unset dmi values

it is not uncommon to find dmi data in /sys full of 'ff'. utf-8
decoding of those would fail, causing warning and stacktrace.

Return '.' instead of \xff. This is what dmidecode would return.

$ dmidecode --string system-product-name

1 files changed, 1 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index da1ca9ee..ebaacf6a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -88,6 +88,7 @@
  - No longer run pollinate in seed_random (LP: #1554152)
  - groups: add defalt user to 'lxd' group.  Create groups listed
    for a user if they do not exist. (LP: #1539317)
+ - dmi data: fix failure of reading dmi data for unset dmi values
 
 0.7.6:
  - open 0.7.6