oox: don't segfault when failing to open a custom fragment

Change-Id: Ie05d1feea2e930b117b3754c6e0db86bdb85bc61
Reviewed-on: https://gerrit.libreoffice.org/51681Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

oox/source/core/xmlfilterbase.cxx

@@ -1118,6 +1118,8 @@ void XmlFilterBase::exportCustomFragments()
         addRelation(customFragmentTypes[j], customFragmentTargets[j]);
         const OUString aFilename = customFragmentTargets[j];
         Reference<XOutputStream> xOutStream = openOutputStream(aFilename);
+        if (xOutStream.is())
+        {
             xOutStream->writeBytes(customFragments[j]);
             uno::Reference<XPropertySet> xProps(xOutStream, uno::UNO_QUERY);
             if (xProps.is())
@@ -1127,6 +1129,7 @@ void XmlFilterBase::exportCustomFragments()
                 xProps->setPropertyValue("MediaType", uno::makeAny(aContentType));
             }
         }
+    }
 }
 
 } // namespace core