Fixed #379.

[redboltz]

Fixed MQTT allowed UTF-8 contents checking.
Fixed reason_code. If invalid UTF-8 detected, the reason_code should be malformed_packet.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 72.00000% with 21 lines in your changes missing coverage. Please review.

Project coverage is 97.59%. Comparing base (455c155) to head (84178c5).

Files with missing lines	Patch %	Lines
include/async_mqtt/util/utf8validate.hpp	71.62%	21 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #381      +/-   ##
==========================================
+ Coverage   97.34%   97.59%   +0.25%     
==========================================
  Files         154      153       -1     
  Lines        9474     9374     -100     
==========================================
- Hits         9222     9149      -73     
+ Misses        252      225      -27     

Files with missing lines	Coverage Δ
include/async_mqtt/packet/detail/base_property.hpp	97.74% <ø> (ø)
include/async_mqtt/packet/impl/v3_1_1_connect.ipp	99.63% <ø> (ø)
include/async_mqtt/packet/impl/v3_1_1_publish.ipp	99.28% <ø> (ø)
...nclude/async_mqtt/packet/impl/v3_1_1_subscribe.ipp	100.00% <ø> (ø)
...lude/async_mqtt/packet/impl/v3_1_1_unsubscribe.ipp	100.00% <ø> (ø)
include/async_mqtt/packet/impl/v5_connect.ipp	99.70% <ø> (ø)
include/async_mqtt/packet/impl/v5_publish.ipp	98.87% <ø> (ø)
include/async_mqtt/packet/impl/v5_subscribe.ipp	99.01% <100.00%> (ø)
include/async_mqtt/packet/impl/v5_unsubscribe.ipp	100.00% <ø> (ø)
include/async_mqtt/util/utf8validate.hpp	71.62% <71.62%> (-28.38%)	⬇️