ps.map: Fix memory leak issue (#4369)

This addresses resource leak issue identified by Coverity Scan (CID : 1207906). Uses Vect_destroy_cats_struct() to destroy Cats and avoid the memory leak issue.

ps/ps.map/ps_vpoints.c

@@ -268,5 +268,6 @@ int PS_vpoints_plot(struct Map_info *P_map, int vec)
     } /* for (line) */
 
     fprintf(PS.fp, "\n");
+    Vect_destroy_cats_struct(Cats);
     return 0;
 }