src/utils.c: Prevent accessing a NULL GHashTable in get_var()

This could happen if we were looking for a query variable when there
were none. We would then see messages like the following in the
error_log.

    (process:14086): GLib-CRITICAL **: g_hash_table_lookup: assertion
    `hash_table != NULL' failed

No real harm was being done, but better to just tidy this up to prevent
such things.

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>