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 maps to what dmidecode would return

$ dmidecode --string system-product-name
.................................

1 files changed, 9 insertions, 0 deletions

diff --git a/tests/unittests/test_util.py b/tests/unittests/test_util.py
index 95990165..542e4075 100644
--- a/tests/unittests/test_util.py
+++ b/tests/unittests/test_util.py
@@ -385,6 +385,15 @@ class TestReadDMIData(helpers.FilesystemMockingTestCase):
         self.patch_mapping({})
         self.assertEqual(None, util.read_dmi_data('expect-fail'))
 
+    def test_dots_returned_instead_of_foxfox(self):
+        my_len = 32
+        dmi_value = b'\xff' * my_len + b'\n'
+        expected = '.' * my_len
+        dmi_key = 'system-product-name'
+        sysfs_key = 'product_name'
+        self._create_sysfs_file(sysfs_key, dmi_value)
+        self.assertEqual(expected, util.read_dmi_data(dmi_key))
+
 
 class TestMultiLog(helpers.FilesystemMockingTestCase):