hashmap: use assert_se() to make clang happy

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.
```