Captive Portal per-user bandwidth input validation fix. Issue #9311

[vktg]

• Redmine Issue: https://redmine.pfsense.org/issues/9311
• Ready for review

From redmine:

I have two routers running on 2.4.4-RELEASE-p2 (amd64) and they have radically different configurations-- one with about 25 active users and the other around 1200 active users. I just took the following steps on both routers to confirm that it's still a problem here:

1. Confirmed in Status/Traffic Graph that several users are exceeding 5M Bits/Sec (some are over 150M Bits/Sec)
2. In Captive Portal profile, Checked "Enabled per-user bandwidth restriction" and set "Default download" to 5000 (left Default upload blank), then saved
3. Confirmed in Status/Traffic Graph that all users' bandwidth has been limited to 5M (confirmed, the top users are sitting at 5)
4. Returned to the Captive Portal profile and Un-Checked "Enabled per-user bandwidth restriction" without clearing the "Default download" blank, then saved
5. Looked in Status/Traffic Graph to see that all users are still being limited to 5M Bits/Sec
6. Returned to the Captive Portal profile, Checked "Enabled per-user bandwidth restriction", then cleared "Default download" blank, then saved
7. In Status/Traffic Graph, users are no longer being restricted (some are over 150M Bits/Sec)

This fix clears <bwdefaultdn> and <bwdefaultup> if <peruserbw> is disabled,
otherwise these values used in

pfsense/src/etc/inc/captiveportal.inc

Line 1302 in a88b841

} else if (!empty($config['captiveportal'][$cpzone]['bwdefaultup'])) {

and

pfsense/src/etc/inc/captiveportal.inc

Line 1308 in a88b841

} else if (!empty($config['captiveportal'][$cpzone]['bwdefaultdn'])) {