ofz#6888/forcepoint#27 adjust consistency check

Change-Id: I0d1f04170bcbc08fe30236e2444761d9d71fb1fb
Reviewed-on: https://gerrit.libreoffice.org/51221Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/source/gdi/regionband.cxx

@@ -1163,6 +1163,8 @@ bool RegionBand::Exclude(const RegionBand& rSource)
 
 bool RegionBand::CheckConsistency() const
 {
+    if (!mpFirstBand)
+        return true;
     // look in the band list (don't test first band again!)
     const ImplRegionBand* pBand = mpFirstBand->mpNextBand;
     while (pBand)