The error handling calls kfree(full_path) so we can't let it be a NULL
pointer.  There used to be a NULL assignment here but we accidentally
deleted it.  Add it back.
Fixes: 7efd08158261 ("cifs: document and cleanup dfs mount")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
                full_path = build_unc_path_to_root(vol, cifs_sb, !!count);
                if (IS_ERR(full_path)) {
                        rc = PTR_ERR(full_path);
+                       full_path = NULL;
                        break;
                }
                /* Chase referral */