don't crash on loading abi13730-1.wpd

Change-Id: I3796f16c4df8cc055fb1a75b1583c5b959f02799
(cherry picked from commit 64a252ff)
Reviewed-on: https://gerrit.libreoffice.org/19243Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: David Tardon <dtardon@redhat.com>

writerperfect/source/writer/WordPerfectImportFilter.cxx

@@ -84,6 +84,9 @@ static bool handleEmbeddedWPGImage(const librevenge::RVNGBinaryData &input, libr
     if (!libwpg::WPGraphics::parse(input.getDataStream(), &aSVGGenerator, fileFormat))
         return false;
 
+    if (svgOutput.empty())
+        return false;
+
     assert(1 == svgOutput.size());
 
     output.clear();