dtoc: Fix Fdt.GetNode() to handle a missing node

At present the algortihm is not correct since it will return the root node
if the requested node is not found and there are no slashes in the
requested node name. Fix this and add a test.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/dtoc/test_fdt.py

@@ -205,6 +205,9 @@ class TestProp(unittest.TestCase):
         self.node = self.dtb.GetNode('/spl-test')
         self.fdt = self.dtb.GetFdtObj()
 
+    def testMissingNode(self):
+        self.assertEqual(None, self.dtb.GetNode('missing'))
+
     def testPhandle(self):
         dtb = fdt.FdtScan('tools/dtoc/dtoc_test_phandle.dts')
         node = dtb.GetNode('/phandle-source')