[dtc] Bugfix for CHECK_HAVE_WARN_PHANDLE()

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> At present, the tree checking code in dtc will die with an assertion
> failure if given a tree which has the invalid value 0 or -1 in a
> property which should contain a phandle.  This patch fixes the check
> to die more gracefully with an error message indicating the invalid
> phandle value.
> 
> Signed-off-by: David Gibson <dwg at au1.ibm.com>

Applied.

Thanks,
jdl