libmso: Use reserve achor in case OfficeArtClientAnchor is missing.

* Sometimes OfficeArtClientAnchor, which provides some client specific data to obtain
shape dimensions and position is missing. Each client should decide what to do
in this case. For MS-DOC client, the data can be accessed based on the character
position (CP) in the document. I could not reproduce the problem after re-save in
MS Office 2000/2003/2007. The test files attached to Bug 275290 is a well known broken file.
I left an assert to obtain test data for inline shapes with missing OfficeArtClientAnchor.