package/atftp: add security fix for CVE-2020-6097

package/atftp: add security fix for CVE-2020-6097

Fixed the following security issue:

- CVE-2020-6097: An exploitable denial of service vulnerability exists in
  the atftpd daemon functionality of atftp 0.7.git20120829-3.1+b1.  A
  specially crafted sequence of RRQ-Multicast requests trigger an assert()
  call resulting in denial-of-service.  An attacker can send a sequence of
  malicious packets to trigger this vulnerability.

For more details, see the report:
https://talosintelligence.com/vulnerability_reports/TALOS-2020-1029

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>