Validate buffer length before reading fields in Packet::readFrom #105

Job	Run time
build (RAK_4631_repeater)	2m 15s
build (RAK_4631_room_server)	2m 11s
build (Heltec_v3_companion_radio_ble)	2m 40s
build (RAK_4631_companion_radio_ble)	2m 15s
build (LilyGo_Tlora_C6_repeater_)	12s
build (Heltec_v3_room_server)	2m 48s
build (PicoW_repeater)	2m 37s
build (wio-e5-mini_repeater)	1m 44s
build (Heltec_v3_repeater)	2m 47s
	19m 29s

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