libfdt: fix UBSAN null pointer in fdt_property()

fdt_property() unconditionally calls memcpy(ptr, val, len) even when
len is zero and val is NULL.  This is a legitimate calling convention
for adding empty FDT properties such as "interrupt-controller", which
carry no payload.

However, compilers that treat memcpy as nonnull on its pointer arguments
will fire UBSAN before observing that len is zero.

Guard the memcpy() with a check on len so it is skipped entirely when
there is no payload to copy, bringing the code in line with the
nonnull contract.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Origin: git://git.kernel.org/pub/scm/utils/dtc/dtc.git f57e7df35df4

Fixes: f0ea065580 ("libfdt: add version 1.3.0")
Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Reviewed-by: Baptiste Le Duc <baptiste.le-duc@vates.tech>
Reviewed-by: Michal Orzel <michal.orzel@amd.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

xen/common/libfdt/fdt_sw.c

@@ -330,7 +330,8 @@ int fdt_property(void *fdt, const char *name, const void *val, int len)
 	ret = fdt_property_placeholder(fdt, name, len, &ptr);
 	if (ret)
 		return ret;
-	memcpy(ptr, val, len);
+	if (len)
+		memcpy(ptr, val, len);
 	return 0;
 }