Improve robustness

`n_valid == 0` occurs if the script is used on the shapefile triplet
of interpolated eddies.

Analysis/stat.py

@@ -75,8 +75,11 @@ print()
 
 print("number of outermost contours (outermost contour found, but may be with "
       "sufficient area or not):", n_outer)
-print("average number of points of outermost contour per valid eddy:",
-      n_points_valid / n_valid)
+
+if n_valid != 0:
+    print("average number of points of outermost contour per valid eddy:",
+          n_points_valid / n_valid)
+    
 print("average number of points of outermost contour per extremum (valid eddy "
       "or not):", n_points / n_extr)
 print()