net/mosquitto: Update to 1.6.12

net/mosquitto: Update to 1.6.12

Security release.  From the changelog:

- In some circumstances, Mosquitto could leak memory when handling PUBLISH
  messages. This is limited to incoming QoS 2 messages, and is related
  to the combination of the broker having persistence enabled, a clean
  session=false client, which was connected prior to the broker restarting,
  then has reconnected and has now sent messages at a sufficiently high rate
  that the incoming queue at the broker has filled up and hence messages are
  being dropped. This is more likely to have an effect where
  max_queued_messages is a small value. This has now been fixed. Closes
  https://github.com/eclipse/mosquitto/issues/1793

Changelog: https://mosquitto.org/blog/2020/08/version-1-6-12-released/

Signed-off-by: Karl Palsson <karlp@etactica.com>