ipn/ipnlocal: fix nil pointer dereference and add related test

Fixes #12644

Change-Id: I3589b01a9c671937192caaedbb1312fd906ca712
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>

ipn/ipnlocal/local.go

@@ -6578,7 +6578,7 @@ func fillAllowedSuggestions() set.Set[tailcfg.StableNodeID] {
 }
 
 func suggestExitNode(report *netcheck.Report, netMap *netmap.NetworkMap, prevSuggestion tailcfg.StableNodeID, selectRegion selectRegionFunc, selectNode selectNodeFunc, allowList set.Set[tailcfg.StableNodeID]) (res apitype.ExitNodeSuggestionResponse, err error) {
-	if report.PreferredDERP == 0 || netMap == nil || netMap.DERPMap == nil {
+	if report == nil || report.PreferredDERP == 0 || netMap == nil || netMap.DERPMap == nil {
 		return res, ErrNoPreferredDERP
 	}
 	candidates := make([]tailcfg.NodeView, 0, len(netMap.Peers))

ipn/ipnlocal/local_test.go

@@ -3118,6 +3118,12 @@ func TestSuggestExitNode(t *testing.T) {
 				DERPMap:  defaultDERPMap,
 			},
 		},
+		{
+			name:       "nil report",
+			lastReport: nil,
+			netMap:     largeNetmap,
+			wantError:  ErrNoPreferredDERP,
+		},
 		{
 			name:       "no preferred derp region",
 			lastReport: preferredNoneReport,
@@ -3127,6 +3133,24 @@ func TestSuggestExitNode(t *testing.T) {
 			},
 			wantError: ErrNoPreferredDERP,
 		},
+		{
+			name:       "nil netmap",
+			lastReport: noLatency1Report,
+			netMap:     nil,
+			wantError:  ErrNoPreferredDERP,
+		},
+		{
+			name:       "nil derpmap",
+			lastReport: noLatency1Report,
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				DERPMap:  nil,
+				Peers: []tailcfg.NodeView{
+					dallasPeer5,
+				},
+			},
+			wantError: ErrNoPreferredDERP,
+		},
 		{
 			name:       "missing suggestion capability",
 			lastReport: noLatency1Report,