libfdt: Remove undefined behaviour setting empty properties

The standard way of setting an empty property using libfdt is:
    fdt_setprop(fdt, nodeoffset, propname, NULL, 0);
However, the implementation of this includes an unconditional:
     memcpy(prop->data, NULL, 0);
Which although it will be a no-op (which is what we want) on many platforms
is technically undefined behaviour.  Correct this, so that when passing
a 0 length, passing a NULL pointer as the value to fdt_setprop() is
definitely safe.  This should quiet static checkers which complain about
this.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>