socksproxy: Fix localhost DNS and use-after-free crash

[oskirby]

Description

In continuing my testing with the socksproxy and the DNS resolution woes, I came across a couple of bugs that can lead to breakage.

The first issue is that when selecting DNS servers, when the first choice is a link-local IPv6 address, we don't set a scope ID in the address, which can prevent c-ares from being able to connect to it. We can fix this just by calling QHostAddress::setScopeId() when building the list of DNS servers. While implementing this, I also performed a bit of cleanup by moving the DNS server selection into WindowsBypass::updateNamserver()

The second issue, which was a bit trickier to track down, is that we have a use-after-free bug in the socks proxy that can occur between the c-ares resolver thread and the Qt thread. This happens when the Socks5Connection is closed before DNS resolution finishes, in which case the arg provided to the c-ares callback points to freed memory. To resolve this, we need to verify that the QObject is valid before invoking the onResolutionFinished() method. We add a QHash map to store the objects that requested resolution, and use the QObject::destroyed() signal to track when they are garabge collected.

Reference

Based upon PR #10317 by @strseb
JIRA Issues:

• VPN-6891
• VPN-6890
• VPN-6874

Checklist

• My code follows the style guidelines for this project
• I have not added any packages that contain high risk or unknown licenses (GPL, LGPL, MPL, etc. consult with DevOps if in question)
• I have performed a self review of my own code
• I have commented my code PARTICULARLY in hard to understand areas
• I have added thorough tests where needed

[mcleinman]

This is far from my area of expertise, but from what I can see it looks good. Thanks!