svx: punish evil-doers who put duplicate properties into custom shapes

LO 4.3.2.2 is evidently able to export an ODF document that violates
XML Well-formedness constraint: Unique Att Spec.

<draw:enhanced-geometry
 draw:mirror-horizontal="false"
 draw:mirror-vertical="false"
 svg:viewBox="0 0 21679 2134682997"
 draw:text-areas="0 0 ?f3 ?f2"
 draw:mirror-vertical="true"
 draw:type="ooxml-rect"
 draw:enhanced-path="M 0 0 L ?f3 0 ?f3 ?f2 0 ?f2 Z N">

Not sure how to reproduce this, but the attributes there are apparently a
serialization of SdrCustomShapeGeometryItem's aPropSeq,
retrieved from a "CustomShapeGeometry" property, so add some input
validation and assertions there.

(cherry picked from commit 7fcbb29db802acd8c0f32e8ff578ef4b2f82c46b)

Backport adapted to remove C++11 lambdas.

Change-Id: I91151365b507779a4bdc9cce2057d34f2376f005
Reviewed-on: https://gerrit.libreoffice.org/13111
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>