Make sure we keep enough space for terminating NUL character after last
newline. If we have too much data, replace last byte with '.'s to
make overflow visible.

Using hex_dump_to_buffer() is not interesting as it adds more overhead
and does not append the trailing linefeed.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>