arm/efi: Fix null pointer dereference

Fix for commit 60649d443d
that introduces a null pointer dereference when the
fdt_node_offset_by_compatible is called with "fdt"
argument null.

Reported-by: Julien Grall <julien@xen.org>
Fixes: 60649d443d ("arm/efi: Introduce xen,uefi-cfg-load DT property")
Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/arch/arm/efi/efi-boot.h

@@ -593,7 +593,8 @@ static bool __init efi_arch_use_config_file(EFI_SYSTEM_TABLE *SystemTable)
     dtbfile.ptr = fdt;
     dtbfile.need_to_free = false; /* Config table memory can't be freed. */
 
-    if ( fdt_node_offset_by_compatible(fdt, 0, "multiboot,module") > 0 )
+    if ( fdt &&
+         (fdt_node_offset_by_compatible(fdt, 0, "multiboot,module") > 0) )
     {
         /* Locate chosen node */
         int node = fdt_subnode_offset(fdt, 0, "chosen");