Zero out freed timers and light sources

src/light.c

@@ -148,6 +148,7 @@ delete_ls(light_source *ls)
     }
     if (curr) {
         assert(curr == ls);
+        (void) memset((genericptr_t) ls, 0, sizeof(light_source));
         free((genericptr_t) ls);
         gv.vision_full_recalc = 1;
     } else {
@@ -433,6 +434,7 @@ save_light_sources(NHFILE *nhfp, int range)
             /* if global and not doing local, or vice versa, remove it */
             if (is_global ^ (range == RANGE_LEVEL)) {
                 *prev = curr->next;
+                (void) memset((genericptr_t) curr, 0, sizeof(light_source));
                 free((genericptr_t) curr);
             } else {
                 prev = &(*prev)->next;