fix: don't panic if no suitable IPv6 src_addr is found

[thvdveld]

Prevent panic when no suitable source address is found. If no suitable address is found, the loopback address is used instead. The function can still panic when the destination address is unspecified.

More tests are added:

• Tests when the interface has no addresses. The loopback address is used as source address.
• Tests when the interface only has a link-local address. The link-local address is used as source address, unless the destination address is the loopback address. In this case, the loopback address is used as source address.

This should fix #894 .

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (d185a37) 79.82% compared to head (d26a313) 79.92%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #895 +/- ## ========================================== + Coverage 79.82% 79.92% +0.09% ========================================== Files 80 80 Lines 28053 28224 +171 ========================================== + Hits 22394 22557 +163 - Misses 5659 5667 +8 ```

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