Commit 17561eb3 modified the logic to check "if (!event)" for an
attribute that was not supposed to be passed as NULL.  This causes
the static checker/Coverity build to fail. Since the check is made,
alter the header.

Also add an error message since returning -1 without some sort of
error message as previously would have happened with the failed
VIR_STRDUP so that the eventual error doesn't get the default
for some reason message.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>