Fix a possible overflow case detected by gcc 4.3.2

.../dtc/libfdt/fdt_sw.c: In function 'fdt_end_node':
.../dtc/libfdt/fdt_sw.c:81: error: assuming signed overflow does not occur when assuming that (X + c) < X is always false

Signed-off-by: Emil Medve <Emilian.Medve@Freescale.com>