This regressed in e17dc362.
g_hash_table_lookup() cannot distinguish between a missing value and a
NULL value. We are storing a NULL pointer (GINT_TO_POINTER (FALSE)) to
indicate that the URL is not a match, so the end result is that instead
of a cache hit indicating we should return FALSE, we instead get a cache
miss and then have to manually determine that we need to return FALSE.

This should be a performance fix only, it should not affect correctness.

Fixes #37