spi_slave: fix crash issue caused by setting cs to -1

Setting cs num to -1 means cs pin is not connected. But passing the value -1 to the
spicommon_cs_initialize() will lead to program crash caused by "array index out
of bounds". We expect the driver to execute well but the driver should not be in
charge of users' behaviour, so this commit prevents the crash.

https://github.com/espressif/esp-idf/issues/5784

components/driver/spi_slave.c

@@ -124,6 +124,7 @@ esp_err_t spi_slave_initialize(spi_host_device_t host, const spi_bus_config_t *b
 #ifndef CONFIG_SPI_SLAVE_ISR_IN_IRAM
     SPI_CHECK((bus_config->intr_flags & ESP_INTR_FLAG_IRAM)==0, "ESP_INTR_FLAG_IRAM should be disabled when CONFIG_SPI_SLAVE_ISR_IN_IRAM is not set.", ESP_ERR_INVALID_ARG);
 #endif
+    SPI_CHECK(slave_config->spics_io_num < 0 || GPIO_IS_VALID_GPIO(slave_config->spics_io_num), "spics pin invalid", ESP_ERR_INVALID_ARG);
 
     spi_chan_claimed=spicommon_periph_claim(host, "spi slave");
     SPI_CHECK(spi_chan_claimed, "host already in use", ESP_ERR_INVALID_STATE);
@@ -151,7 +152,10 @@ esp_err_t spi_slave_initialize(spi_host_device_t host, const spi_bus_config_t *b
         ret = err;
         goto cleanup;
     }
-    spicommon_cs_initialize(host, slave_config->spics_io_num, 0, !bus_is_iomux(spihost[host]));
+    if (slave_config->spics_io_num >= 0) {
+        spicommon_cs_initialize(host, slave_config->spics_io_num, 0, !bus_is_iomux(spihost[host]));
+    }
+
     // The slave DMA suffers from unexpected transactions. Forbid reading if DMA is enabled by disabling the CS line.
     if (use_dma) freeze_cs(spihost[host]);