riscv: ae350: Check firmware_fdt_addr header

Check firmware_fdt_addr header to see if it
is a valid fdt blob.

Signed-off-by: Rick Chen <rick@andestech.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
diff --git a/board/AndesTech/ax25-ae350/ax25-ae350.c b/board/AndesTech/ax25-ae350/ax25-ae350.c
index 36f0dd4..63a966e 100644
--- a/board/AndesTech/ax25-ae350/ax25-ae350.c
+++ b/board/AndesTech/ax25-ae350/ax25-ae350.c
@@ -60,7 +60,7 @@
 	*err = 0;
 
 	if (IS_ENABLED(CONFIG_OF_SEPARATE) || IS_ENABLED(CONFIG_OF_BOARD)) {
-		if (gd->arch.firmware_fdt_addr)
+		if (fdt_magic((uintptr_t)gd->arch.firmware_fdt_addr) == FDT_MAGIC)
 			return (void *)(ulong)gd->arch.firmware_fdt_addr;
 	}