FOF hasmap crashes when re-hashing

As discussed in !543 (merged), the FOF code sometimes crashes when the re-hasing of the has table creates a collision.

The relevant line of code is

          if (!new_element) {
            /* TODO(pedro): Deal with this type of failure more elegantly. */
            error("Failed to re-hash element.");
          }

Unfortunately this corner case does happen in real-life runs.

Also, we may want to use a larger initial table size to prevent spending too much time growing it.