df: fix mount list processing with unstatable mount dirs

* src/df.c (filter_mount_list): Initialize devlist->dev_num correctly
when unable to stat() a mount point.  This will avoid possible invalid
deduplication done on the list due to use of uninitialized memory.
* tests/df/skip-duplicates.sh: Ensure this code path is exercised.
Also refactor the test to be table driven.
* NEWS: Mention the bug fix.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/df.c b/src/df.c
index 051513196..bc507c72e 100644
--- a/src/df.c
+++ b/src/df.c
@@ -620,7 +620,8 @@ filter_mount_list (void)
 
       if (-1 == stat (me->me_mountdir, &buf))
         {
-          ;  /* Stat failed - add ME to be able to complain about it later.  */
+          /* Stat failed - add ME to be able to complain about it later.  */
+          buf.st_dev = me->me_dev;
         }
       else
         {