don't hang on busted subdocument offsets

Change-Id: I5d1d18f864496ec98894703f0a22e211a9dd14a5
(cherry picked from commit a2b90cd0)
Reviewed-on: https://gerrit.libreoffice.org/18747Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>

sw/source/filter/ww8/ww8scan.cxx

@@ -4450,10 +4450,14 @@ void WW8PLCFxDesc::ReduceByOffset()
         else
             nStartPos -= nCpOfs;
     }
-    if( nEndPos != WW8_CP_MAX )
+    if (nEndPos != WW8_CP_MAX)
     {
-        OSL_ENSURE(nCpOfs <= nEndPos,
-            "oh oh, so much for the subdocument piece theory");
+        if (nCpOfs > nEndPos)
+        {
+            SAL_WARN("sw.ww8", "broken subdocument piece entry");
+            nEndPos = WW8_CP_MAX;
+        }
+        else
             nEndPos -= nCpOfs;
     }
 }