change spl_header @ 0x0 -> spl_signature @ 0x4 to avoid dereferencing NULL

uart0-helloworld-sdboot.diff

diff --git a/uart0-helloworld-sdboot.c b/uart0-helloworld-sdboot.c
index 2278b0b..0d62cf0 100644
--- a/uart0-helloworld-sdboot.c
+++ b/uart0-helloworld-sdboot.c
@@ -419,16 +419,18 @@ enum { BOOT_DEVICE_UNK, BOOT_DEVICE_FEL, BOOT_DEVICE_MMC0, BOOT_DEVICE_SPI };
 
 int get_boot_device(void)
 {
-	u32 *spl_header = (void *)0x0;
+	u32 *spl_signature = (void *)0x4;
 	if (soc_is_a64() || soc_is_a80())
-		spl_header = (void *)0x10000;
+		spl_signature = (void *)0x10004;
 
 	/* Check the eGON.BT0 magic in the SPL header */
-	if (!(spl_header[1] == 0x4E4F4765 && spl_header[2] == 0x3054422E))
+	if (spl_signature[0] != 0x4E4F4765 || spl_signature[1] != 0x3054422E)
 		return BOOT_DEVICE_FEL;
-	else if ((spl_header[10] & 0xFF) == 0)
+
+	u32 boot_dev = spl_signature[9] & 0xFF; /* offset into SPL = 0x28 */
+	if (boot_dev == 0)
 		return BOOT_DEVICE_MMC0;
-	else if ((spl_header[10] & 0xFF) == 3)
+	if (boot_dev == 3)
 		return BOOT_DEVICE_SPI;
 
 	return BOOT_DEVICE_UNK;