efi/fdt: fix panic when no valid fdt found

setup_arch() would invoke efi_init()->efi_get_fdt_params(). If no
valid fdt found then initial_boot_params will be null. So we
should stop further fdt processing here. I encountered this
issue on risc-v.
Signed-off-by: Changbin Du <changbin.du@gmail.com>
Fixes: b91540d5 ("RISC-V: Add EFI runtime services")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/fdtparams.c

@@ -98,6 +98,9 @@ u64 __init efi_get_fdt_params(struct efi_memory_map_data *mm)
 	BUILD_BUG_ON(ARRAY_SIZE(target) != ARRAY_SIZE(name));
 	BUILD_BUG_ON(ARRAY_SIZE(target) != ARRAY_SIZE(dt_params[0].params));
 
+	if (!fdt)
+		return 0;
+
 	for (i = 0; i < ARRAY_SIZE(dt_params); i++) {
 		node = fdt_path_offset(fdt, dt_params[i].path);
 		if (node < 0)