WiFiFirmwareUpdater: restore null termination #1362

Job	Run time
compile-examples (arduino:mbed:envie_m7, envie_m7)	25m 7s
compile-examples (arduino:mbed:envie_m7:target_core=cm4, envie_m7_core_cm4)	5m 9s
compile-examples (arduino:mbed:opta, opta)	15m 31s
compile-examples (arduino:mbed:nicla_sense, nicla_sense)	10m 57s
compile-examples (arduino:mbed:edge_control, edge_control)	4m 50s
compile-examples (arduino:mbed:nanorp2040connect, nanorp2040connect)	11m 46s
compile-examples (arduino:mbed:nicla_voice, nicla_voice)	4m 47s
compile-examples (arduino:mbed:giga, giga)	24m 24s
compile-examples (arduino:mbed:nicla_vision, nicla_vision)	13m 36s
compile-examples (arduino:mbed:nano33ble, nano33ble)	7m 4s
	2h 3m 11s

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