fix bug in contour scanner

extensions/fablabchemnitz/contour_scanner_and_trimmer/contour_scanner_and_trimmer.py

@@ -859,6 +859,7 @@ class ContourScannerAndTrimmer(inkex.EffectExtension):
 
         if so.remove_subsplit_collinear is True:
             if so.show_debug is True: self.msg("filtering collinear overlapping lines / duplicate lines")
+            if len(subSplitLineArray) > 0:
                 output_set, dropped_ids = self.filter_collinear(subSplitLineArray)
                 for subSplitLine in subSplitLineArray:
                     ssl_id = subSplitLine.get('id')