Validate buffer length before reading fields in Packet::readFrom #673

Job	Run time
build (RAK_4631_room_server)	2m 16s
build (Heltec_v3_companion_radio_ble)	2m 56s
build (Heltec_v3_room_server)	2m 52s
build (RAK_4631_repeater)	2m 11s
build (Heltec_v3_repeater)	2m 44s
build (RAK_4631_companion_radio_ble)	2m 31s
build (wio-e5-mini_repeater)	1m 29s
build (LilyGo_Tlora_C6_repeater_)	2m 0s
build (PicoW_repeater)	2m 48s
	21m 47s

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