abi#10076 RTF export: handle frames without a valid anchor point

Ideally all frames have an anchor point in the maFrames vector, as
provided by MSWordExportBase. If that's not the case, then we have no
idea what would be a valid anchor point: just export those as inline, as
we did before.

Change-Id: I81134d9fb8dc4ca166084964d277ac02ceccde7b