fix assert on loading kde98819-1.html

Change-Id: I3f8ccf2d0642657b3768a237c96f15cf7a7aff97
(cherry picked from commit 1cc39606)

ucb/source/ucp/webdav-neon/NeonSession.cxx

@@ -349,7 +349,9 @@ namespace {
         {
             nContStart += sPartId.getLength();
             sal_Int32 nContEnd = _rRawString.indexOf( ',', nContStart );
-            sPart = _rRawString.copy( nContStart, nContEnd - nContStart );
+            sPart = nContEnd != -1 ?
+                _rRawString.copy(nContStart, nContEnd - nContStart) :
+                _rRawString.copy(nContStart);
         }
         return sPart;
     }