Do not find new dependency IDs when there are unconnected p: tags

Suppose the task:

Some item p:1

(there is no parent). Then the next time a new ID is selected, it would
return 1 because there is no id: tag with value 1. However, this will
result in an invalid dependency.

test/test_todo_list.py

@@ -249,6 +249,7 @@ class TodoListDependencyTester(TopydoTest):
         self.todolist.add("Another one with +Project")
         self.todolist.add("Todo with +AnotherProject")
         self.todolist.add("Todo without children id:3")
+        self.todolist.add("Orphan p:4")
 
     def test_check_dep(self):
         children = self.todolist.children(self.todolist.todo(1))
@@ -279,8 +280,8 @@ class TodoListDependencyTester(TopydoTest):
         todo5 = self.todolist.todo(5)
         self.todolist.add_dependency(todo5, todo4)
 
-        self.assertTrue(todo5.has_tag('id', '4'))
-        self.assertTrue(todo4.has_tag('p', '4'))
+        self.assertTrue(todo5.has_tag('id', '5'))
+        self.assertTrue(todo4.has_tag('p', '5'))
 
     def test_add_dep2(self):
         """
@@ -294,8 +295,8 @@ class TodoListDependencyTester(TopydoTest):
         self.todolist.add_dependency(todo5, todo4)
         self.todolist.add_dependency(todo4, todo1)
 
-        self.assertTrue(todo4.has_tag('id', '5'))
-        self.assertTrue(todo1.has_tag('p', '5'))
+        self.assertTrue(todo4.has_tag('id', '6'))
+        self.assertTrue(todo1.has_tag('p', '6'))
 
     def test_add_dep3(self):
         """

topydo/lib/TodoList.py

@@ -117,7 +117,8 @@ class TodoList(TodoListBase):
                 Returns True if there exists a todo with the given parent ID.
                 """
                 for todo in self._todos:
-                    if todo.has_tag('id', str(p_id)):
+                    number = str(p_id)
+                    if todo.has_tag('id', number) or todo.has_tag('p', number):
                         return True
 
                 return False