client: fixed a bug where corrupt client state could panic the client (#19972)

.changelog/19972.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+client: Fixed a bug where corrupt client state could panic the client
+```

client/client_test.go

@@ -1876,6 +1876,14 @@ func TestClient_hasLocalState(t *testing.T) {
 
 	c.stateDB = cstate.NewMemDB(c.logger)
 
+	t.Run("nil Job", func(t *testing.T) {
+		alloc := mock.BatchAlloc()
+		alloc.Job = nil
+		c.stateDB.PutAllocation(alloc)
+
+		must.False(t, c.hasLocalState(alloc))
+	})
+
 	t.Run("plain alloc", func(t *testing.T) {
 		alloc := mock.BatchAlloc()
 		c.stateDB.PutAllocation(alloc)

nomad/structs/structs.go

@@ -4770,6 +4770,9 @@ func (j *Job) Warnings() error {
 
 // LookupTaskGroup finds a task group by name
 func (j *Job) LookupTaskGroup(name string) *TaskGroup {
+	if j == nil {
+		return nil
+	}
 	for _, tg := range j.TaskGroups {
 		if tg.Name == name {
 			return tg