Ethernet OPTA: fix crash when DHCP is used on fast DHCP server #1124

Job	Run time
compile-examples (arduino:mbed:nano33ble)	7m 35s
compile-examples (arduino:mbed:envie_m7)	17m 8s
compile-examples (arduino:mbed:nanorp2040connect)	12m 57s
compile-examples (arduino:mbed:nicla_sense)	7m 34s
compile-examples (arduino:mbed:nicla_vision)	12m 29s
compile-examples (arduino:mbed:opta)	24m 51s
compile-examples (arduino:mbed:giga)	24m 27s
compile-examples (arduino:mbed:nicla_voice)	5m 37s
compile-examples (arduino:mbed:edge_control)	5m 47s
compile-examples (arduino:mbed:envie_m7:target_core=cm4, - libraries/KernelDebug - libraries/Port...	4m 38s
	2h 3m 3s

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