This patch fixes breakage of of_find_node_by_path when
node not found in tree. This fault is caused, for example,
when no aliases is present in dtb.

barebox@ConnectCore i.MX51:/ oftree -d /env/mm.dtb
/ {
        model = "Freescale i.MX51 CCMX51";
        compatible = "fsl,imx51-ccmx51", "fsl,imx51";
        chosen {
        };
        cpus {
                cpu@0 {
                        compatible = "arm,cortex-a8";
                };
        };
};
barebox@ConnectCore i.MX51:/ oftree -p /env/mm.dtb
unable to handle NULL pointer dereference at address 0x00000000
pc : [<97f1f3f0>]    lr : [<97f19438>]
sp : 95eff988  ip : 00000001  fp : 97f43080
r10: 00000000  r9 : 97f4304c  r8 : 00000000
r7 : 00000000  r6 : e3510034  r5 : 00000001  r4 : 95f0eb28
r3 : 97f43054  r2 : 00000000  r1 : e1a0000b  r0 : 00000000
Flags: nzCv  IRQs off  FIQs off  Mode SVC_32

no stack data available

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>