powerpc: Fix boot wrapper memcmp() called with zero length argument

I noticed, when trying to use, e.g.,
node = find_node_by_prop_value(prev, "booleanprop", "", 0))
to search for all nodes with a certain boolean property, that memcmp()
returns garbage when comparing zero bytes.  It should return zero.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>