mso-test: element p:sldSz has invalid cx and cy attribute

found by converting tdf43054--1.odp to pptx and running officeotron on the output

Validating part "/ppt/presentation.xml" using schema "29500T/pml.xsd" ...
- (ppt/presentation.xml:2 col:1,116) cvc-minInclusive-valid:
Value '900113' is not facet-valid with respect to minInclusive '914400' for type 'ST_SlideSizeCoordinate'.

We get the slide size from the master page, and this page appears not to have any
width or height values at all, so use a useful value in that case.

Change-Id: I3624b4ba79ec4919655995c20d729b22ff443641
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/194701
Reviewed-by: Michael Stahl <michael.stahl@collabora.com>
Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com>
(cherry picked from commit 2d82d56d3f536ba2bbe0496c4f6488e61a2247cb)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/194752
Tested-by: Jenkins

xmloff/source/draw/ximpstyl.cxx

@@ -27,6 +27,7 @@
 #include <xmloff/xmluconv.hxx>
 #include "ximpnote.hxx"
 #include <xmlsdtypes.hxx>
+#include <tools/gen.hxx>
 #include <tools/debug.hxx>
 #include <sal/log.hxx>
 #include <comphelper/diagnose_ex.hxx>
@@ -358,6 +359,12 @@ SdXMLPageMasterStyleContext::SdXMLPageMasterStyleContext(
                 XMLOFF_WARN_UNKNOWN("xmloff", aIter);
         }
     }
+    // Set size to useful default value, same as in SdDrawDocument::CreateFirstPages
+    // except we cannot call SvxPaperInfo::GetDefaultPaperSize from here.
+    if (mnWidth == 0)
+        mnWidth = 21000;
+    if (mnHeight == 0)
+        mnHeight = 29700;
 }
 
 SdXMLPageMasterStyleContext::~SdXMLPageMasterStyleContext()