Fix segfault in model.Tree(procs)

Fixes crash in model.Tree(procs) which occurs when the list of procs
passed, isn't in the expected order. E.g. if the first element doesn't
have `PPID == 0`, it would crash writing to the uninitialized
`parent.Children`.

model/proc.go

@@ -46,14 +46,15 @@ func Tree(procs []*Proc) []*Proc {
 	var (
 		nodes    []*Proc
 		parent   *Proc
+		children []*Proc
 	)
 	for _, proc := range procs {
 		if proc.PPID == 0 {
 			nodes = append(nodes, proc)
 			parent = proc
-			continue
+			parent.Children = children
 		} else {
-			parent.Children = append(parent.Children, proc)
+			children = append(children, proc)
 		}
 	}
 	return nodes