net/netcheck: don't panic if a region has no Nodes

Updates #13728

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I1e8319d6b2da013ae48f15113b30c9333e69cc0b

net/netcheck/netcheck.go

@@ -503,6 +503,10 @@ func makeProbePlanInitial(dm *tailcfg.DERPMap, ifState *netmon.State) (plan prob
 	plan = make(probePlan)
 
 	for _, reg := range dm.Regions {
+		if len(reg.Nodes) == 0 {
+			continue
+		}
+
 		var p4 []probe
 		var p6 []probe
 		for try := 0; try < 3; try++ {