spi: stm32: fix sram pool free in probe error path

Add a test to check whether the sram_pool is NULL before freeing it.

Fixes: d17dd2f ("spi: stm32: use STM32 DMA with STM32 MDMA to enhance DDR use")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Alain Volmat <alain.volmat@foss.st.com>
Signed-off-by: Clément Le Goffic <clement.legoffic@foss.st.com>
Link: https://patch.msgid.link/20250630-spi-fix-v2-1-4680939e2a3e@foss.st.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: clegoffic

drivers/spi/spi-stm32.c

@@ -2486,7 +2486,9 @@ static int stm32_spi_probe(struct platform_device *pdev)
 	if (spi->mdma_rx)
 		dma_release_channel(spi->mdma_rx);
 err_pool_free:
-	gen_pool_free(spi->sram_pool, (unsigned long)spi->sram_rx_buf, spi->sram_rx_buf_size);
+	if (spi->sram_pool)
+		gen_pool_free(spi->sram_pool, (unsigned long)spi->sram_rx_buf,
+			      spi->sram_rx_buf_size);
 err_dma_release:
 	if (spi->dma_tx)
 		dma_release_channel(spi->dma_tx);