www/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>
www/url_handlers.c: Fix password changing checks

When a user is changing their details we check to see if they are
changing their password, we do this by looking at the password fields to
see if they are empty or not. However the logic here was incorrect, we
were checking for non-empty fields like

    password1 && password2

However that would actually allow you to enter anything into one of the
password fields while leaving the other empty and could lead to your
password being set to what you typed, even if it was too short.

The fixed loigc is

    password1 || password2

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