Fix NULL pointer dereference in STM32 SPI construct #17936

Job	Run time
scheduler	49s
mpy-cross-mac	32s
mpy-cross / build (static-aarch64)	37s
mpy-cross / build (static-mingw)	1m 9s
mpy-cross / build (static)	20s
tests / run (native)	45s
mpy-cross / build (static-raspbian)	50s
tests / run (mpy)	37s
tests / zephyr	17m 5s
tests / run (native_mpy)	39s
tests / run (all)	58s
docs	0s
windows-zephyr	0s
windows	0s
ports (stm) / board (daisy_seed_with_sdram)	1m 51s
ports (stm) / board (openmv_h7)	1m 48s
ports (stm) / board (pyboard_v11)	1m 57s
ports (stm) / board (pyb_nano_v2)	1m 32s
ports (stm) / board (espruino_wifi)	2m 1s
ports (stm) / board (nucleo_f767zi)	2m 6s
ports (stm) / board (meowbit_v121)	3m 12s
ports (stm) / board (nucleo_h743zi_2)	1m 55s
ports (stm) / board (stm32f411ce_blackpill_with_flash)	1m 39s
ports (stm) / board (feather_stm32f405_express)	2m 15s
ports (stm) / board (blues_cygnet)	4m 55s
ports (stm) / board (sparkfun_stm32f405_micromod)	2m 4s
ports (stm) / board (nucleo_f446re)	1m 24s
ports (stm) / board (sparkfun_stm32_thing_plus)	1m 50s
ports (stm) / board (espruino_pico)	1m 38s
ports (stm) / board (stm32f411ce_blackpill)	1m 36s
ports (stm) / board (stm32f412zg_discovery)	1m 56s
ports (stm) / board (stm32f411ve_discovery)	1m 47s
ports (stm) / board (nucleo_f746zg)	2m 12s
ports (stm) / board (swan_r5)	1m 55s
ports (stm) / board (stm32f4_discovery)	1m 51s
ports (stm) / board (stm32f746g_discovery)	2m 4s
ports (stm) / board (thunderpack_v11)	1m 29s
ports (stm) / board (thunderpack_v12)	1m 47s
	1h 13m 5s

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