net/mosquitto: Update to 2.0.20

Changes since 2.0.18:

 - Fix mismatched subscribe/unsubscribe with normal/shared topics.

 - Fix crash on bridge using remapped topic being sent a crafted
   packet.

 - Fix assert failure when loading a persistence file that contains
   subscriptions with no client id.

 - Fix local bridges being incorrectly expired when
   persistent_client_expiration is in use.

 - Fix use of CLOCK_BOOTTIME for getting time.

 - Fix mismatched subscribe/unsubscribe with normal/shared topics.

 - Fix crash on bridge using remapped topic being sent a crafted
   packet.

 - Fix some error codes being converted to string as "unknown".

 - Clear SSL error state to avoid spurious error reporting.

 - Fix "payload format invalid" not being allowed as a PUBREC reason
   code.

 - Don't allow SUBACK with missing reason codes.

 - Fix QoS 1 / QoS 2 publish incorrectly returning "no subscribers".

 - Don't allow invalid response topic values.

 - Fix some strict protocol compliance issues.

PR:		283395
Reported by:	ari@stonepile.fi
Approved by:	joe@thrallingpenguin.com (maintainer timeout, 5 weeks)