Fix sockets not closing properly by connected()

[indutny]

When checking client.connected() it would set _sock to 255 internally, but won't actually communicate back to NINA that the fd is closed so it won't be able to be reused.

See also: adafruit#18

[CLAassistant]

All committers have signed the CLA.

[indutny]

Force pushed with my github email and signed the CLA.

[github-actions]

Memory usage change @ 64b0250

Board	flash	%	RAM for global variables	%
arduino:mbed_nano:nanorp2040connect	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:samd:mkrvidor4000	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:samd:mkrwifi1010	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:samd:nano_33_iot	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0

Click for full report table

Board	examples/PreferencesCounter flash	%	examples/PreferencesCounter RAM for global variables	%	examples/PreferencesValidation flash	%	examples/PreferencesValidation RAM for global variables	%
arduino:mbed_nano:nanorp2040connect	0	0.0	0	0.0	0	0.0	0	0.0
arduino:samd:mkrvidor4000	0	0.0	0	0.0	0	0.0	0	0.0
arduino:samd:mkrwifi1010	0	0.0	0	0.0	0	0.0	0	0.0
arduino:samd:nano_33_iot	0	0.0	0	0.0	0	0.0	0	0.0

Click for full report CSV

Board,examples/PreferencesCounter<br>flash,%,examples/PreferencesCounter<br>RAM for global variables,%,examples/PreferencesValidation<br>flash,%,examples/PreferencesValidation<br>RAM for global variables,%
arduino:mbed_nano:nanorp2040connect,0,0.0,0,0.0,0,0.0,0,0.0
arduino:samd:mkrvidor4000,0,0.0,0,0.0,0,0.0,0,0.0
arduino:samd:mkrwifi1010,0,0.0,0,0.0,0,0.0,0,0.0
arduino:samd:nano_33_iot,0,0.0,0,0.0,0,0.0,0,0.0