conf: Avoid double free in virDomainEventTunableNew callers

virDomainEventTunableNew is supposed to consume and free @params, but it
failed to always set @params to NULL to make sure the caller doesn't try
to free the same memory again.

Fixes: d95c79fbd0
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

src/conf/domain_event.c

@@ -1515,6 +1515,7 @@ virDomainEventTunableNew(int id,
 
  error:
     virTypedParamsFree(*params, nparams);
+    *params = NULL;
     return NULL;
 }