libfdt: Add missing RW_CHECK_HEADER to fdt_del_node()

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> fdt_del_node(), unlike most of the rw functions does not check the
> fdt's header with RW_CHECK_HEADER.  However, it could make a mess of
> things if the conditions in RW_CHECK_HEADER aren't met.  So, this
> patch adds the omitted check.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl