Don't return a value from a void function

Author: erayd

src/libec/misc/skiplist.c

@@ -131,7 +131,7 @@ int ec_sl_set(ec_sl_t *l, void *key, void *data, ec_sl_freefn_t freefn) {
  */
 void ec_sl_remove(ec_sl_t *l, void *key, ec_sl_freefn_t freefn) {
   if(!key)
-    return NULL;
+    return;
   ec_sl_cursor_t c;
   ec_sl_cursor(l, &c, key);
   ec_sl_node_t *n = c[1]->next;