libfdt: Fix bugs in fdt_get_path()

The current implementation of fdt_get_path() has a couple of bugs,
fixed by this patch.

First, contrary to its documentation, on success it returns the length
of the node's path, rather than 0.  The testcase is correspondingly
wrong, and the patch fixes this as well.

Second, in some circumstances, it will return -FDT_ERR_BADOFFSET
instead of -FDT_ERR_NOSPACE when given insufficient buffer space.
Specifically this happens when there is insufficient space even to
hold the path's second last component.  This behaviour is corrected,
and the testcase updated to check it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>