fdt: fix 'prop (...) not found!' error in 'fdt set' command

This commit brings things back to the well known working state of the
command.
-
With commit 9620d872
(cmd/fdt: support single value replacement within an array)

there was an error introduced modifying (inserting) a property to a
device-tree node.
fdt_getprop(...) returnes a len with -1 for a non-existing property, but
a memcpy with len -1 isn't a good idea and things went wrong (crash).
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Some times later Tom did repair this
with commit 99bb38e2
(fdt: Check for NULL return from fdt_getprop in 'fdt set')

This repairs the crash but the behaviour of the command isn't like
before, it makes it impossible to insert a property.
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Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Acked-by: Simon Glass <sjg@chromium.org>