Although the IEnumFORMATETC::Next documentation only mentions S_OK and
S_FALSE, there are error codes that also may be returned: e.g.,
0x800706BA RPC server unavailable (encountered locally in a unit test).
In that case, this used to loop infinitely.

We are only interested in S_OK result, so check for it.

Change-Id: I062e409e84efb68353321f7c48d922ec83191f73
Reviewed-on: https://gerrit.libreoffice.org/61621
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>