Ethernet OPTA: fix crash when DHCP is used on fast DHCP server #1126

Job	Run time
compile-examples (arduino:mbed:nano33ble)	7m 44s
compile-examples (arduino:mbed:envie_m7)	16m 40s
compile-examples (arduino:mbed:nanorp2040connect)	13m 2s
compile-examples (arduino:mbed:nicla_sense)	7m 42s
compile-examples (arduino:mbed:nicla_vision)	11m 59s
compile-examples (arduino:mbed:opta)	25m 22s
compile-examples (arduino:mbed:giga)	25m 7s
compile-examples (arduino:mbed:nicla_voice)	5m 32s
compile-examples (arduino:mbed:edge_control)	5m 43s
compile-examples (arduino:mbed:envie_m7:target_core=cm4, - libraries/KernelDebug - libraries/Port...	4m 24s
	2h 3m 15s

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