dtc: fix for_each_*() to skip first object if deleted

The previous definition of for_each_*() would always include the very
first object within the list, irrespective of whether it was marked
deleted, since the deleted flag was not checked on the first object,
but only on any "next" object.

Fix for_each_*() to check the deleted flag in the loop body every
iteration to correct this.

Incidentally, this change is why commit 45013d86 dtc: "Add ability to
delete nodes and properties" only caused two "make checkm" failures;
only two tests actually use multiple labels on the same property or
node. With this current change applied, but commit 317a5d92 "dtc: zero
out new label objects" reverted, "make checkm" fails 29 times; i.e.
for every test that uses any labels at all.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>