Make empty paths result in FDT_ERR_BADPATH.
Per the specification (v0.4-rc4):
> The convention for specifying a device path is:
> /node-name-1/node-name-2/node-name-N
>
> The path to the root node is /.
>
> A unit address may be omitted if the full path to the
> node is unambiguous.
Bug: 246465319
Test: libfdt_fuzzer # clusterfuzz/testcase-detail/4530863420604416
Change-Id: I14ab0a074ab994c1f598243d2d5795d2cd9a853a
(cherry picked from commit 3c28f3e3a1)
d10c84c4bc