hashmap: use assert_se() to make clang happy
    
    Otherwise it complains about a set but unused variable:
    
    ```
    ../src/basic/hashmap.c:1070:48: error: variable 'n_rehashed' set but not used [-Werror,-Wunused-but-set-variable]
            unsigned old_n_buckets, new_n_buckets, n_rehashed, new_n_entries;
                                                           ^
                                                           1 error generated.
    ```