libfdt: undef sanitizer fix fdt_setprop()

Fixes undefined sanitizer problem in fdt_setprop().

The compiler (gcc 5.3) issues some runtime checks with
-fsanitize=undefined which is triggered if for instance any of the
pointers supplied to memcpy() is NULL as val can be in fdt_setprop() if
there's no value.

Reviewed-by: Jerome Forissier <[email protected]>
Tested-by: Jens Wiklander <[email protected]> (QEMUv7)
Signed-off-by: Jens Wiklander <[email protected]>