fwk: Zero event list node when duplicating

Duplicating events does not currently zero the list node. This triggers
some of the debug safety checks in the list component when trying to
push the event to a new list, as it will refuse to operate on a node
that it thinks is still part of another list.

Change-Id: Ida83f45f2fdbd1dd4042ca09633a1bc90903de6d
Signed-off-by: Chris Kay <chris.kay@arm.com>