Fix NULL pointer dereference in STM32 SPI construct

Move mark_deinit() before check_pins() so that self->sck is not
NULLed after check_pins sets it. The previous ordering caused a
NULL dereference of self->sck->altfn_index, crashing all STM32
boards that use SPI (including STM32F405 boards whose CIRCUITPY
filesystem lives on external SPI flash).

Fixes #10866

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 2 people

ports/stm/common-hal/busio/SPI.c

@@ -136,12 +136,13 @@ void common_hal_busio_spi_construct(busio_spi_obj_t *self,
     const mcu_pin_obj_t *sck, const mcu_pin_obj_t *mosi,
     const mcu_pin_obj_t *miso, bool half_duplex) {
 
+    // Ensure the object starts in its deinit state before check_pins sets
+    // self->sck, self->mosi, and self->miso.
+    common_hal_busio_spi_mark_deinit(self);
+
     int periph_index = check_pins(self, sck, mosi, miso);
     SPI_TypeDef *SPIx = mcu_spi_banks[periph_index - 1];
 
-    // Ensure the object starts in its deinit state.
-    common_hal_busio_spi_mark_deinit(self);
-
     // Start GPIO for each pin
     GPIO_InitTypeDef GPIO_InitStruct = {0};
     GPIO_InitStruct.Pin = pin_mask(sck->number);