ipn/ipnlocal: don't panic if there are no suitable exit nodes

In suggestExitNodeLocked, if no exit node candidates have a home DERP or
valid location info, `bestCandidates` is an empty slice. This slice is
passed to `selectNode` (`randomNode` in prod):

```go func randomNode(nodes views.Slice[tailcfg.NodeView], …) tailcfg.NodeView {
	…
	return nodes.At(rand.IntN(nodes.Len()))
}
```

An empty slice becomes a call to `rand.IntN(0)`, which panics.

This patch changes the behaviour, so if we've filtered out all the
candidates before calling `selectNode`, reset the list and then pick
from any of the available candidates.

This patch also updates our tests to give us more coverage of `randomNode`,
so we can spot other potential issues.

Updates #17661

Change-Id: I63eb5e4494d45a1df5b1f4b1b5c6d5576322aa72
Signed-off-by: Alex Chan <alexc@tailscale.com>

0 files changed, 0 insertions, 0 deletions