WiFiFirmwareUpdater: restore null termination #1363

Job	Run time
compile-examples (arduino:mbed:nicla_vision, nicla_vision)	13m 46s
compile-examples (arduino:mbed:nicla_sense, nicla_sense)	11m 10s
compile-examples (arduino:mbed:nanorp2040connect, nanorp2040connect)	12m 2s
compile-examples (arduino:mbed:envie_m7, envie_m7)	24m 56s
compile-examples (arduino:mbed:opta, opta)	15m 34s
compile-examples (arduino:mbed:edge_control, edge_control)	4m 54s
compile-examples (arduino:mbed:envie_m7:target_core=cm4, envie_m7_core_cm4)	5m 17s
compile-examples (arduino:mbed:giga, giga)	25m 54s
compile-examples (arduino:mbed:nicla_voice, nicla_voice)	4m 39s
compile-examples (arduino:mbed:nano33ble, nano33ble)	7m 8s
	2h 5m 20s

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