libfdt: Add missing RW_CHECK_HEADER to fdt_del_node()

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@gibson.dropbear.id.au>

libfdt/fdt_rw.c

@@ -329,6 +329,8 @@ int fdt_del_node(void *fdt, int nodeoffset)
 {
 	int endoffset;
 
+	RW_CHECK_HEADER(fdt);
+
 	endoffset = _fdt_node_end_offset(fdt, nodeoffset);
 	if (endoffset < 0)
 		return endoffset;