Fix divison-by-zero when adding requirement to dynamic challenge (#782)

* Fixing a bug where prerequisites could not be set for dynamic challenges due to a division by zero error where defaults were being set unnecessarily. 
* Creating unit test for adding requirement to dynamic challenges

tests/challenges/test_dynamic.py

@@ -85,6 +85,63 @@ def test_can_update_dynamic_challenge():
     destroy_ctfd(app)
 
 
+def test_can_add_requirement_dynamic_challenge():
+    """Test that requirements can be added to dynamic challenges"""
+    app = create_ctfd(enable_plugins=True)
+    with app.app_context():
+        challenge_data = {
+            "name": "name",
+            "category": "category",
+            "description": "description",
+            "value": 100,
+            "decay": 20,
+            "minimum": 1,
+            "state": "hidden",
+            "type": "dynamic"
+        }
+        req = FakeRequest(form=challenge_data)
+        challenge = DynamicValueChallenge.create(req)
+
+        assert challenge.value == 100
+        assert challenge.initial == 100
+        assert challenge.decay == 20
+        assert challenge.minimum == 1
+
+        challenge_data = {
+            "name": "second_name",
+            "category": "category",
+            "description": "new_description",
+            "value": "200",
+            "initial": "200",
+            "decay": "40",
+            "minimum": "5",
+            "max_attempts": "0",
+            "state": "visible"
+        }
+
+        req = FakeRequest(form=challenge_data)
+        challenge = DynamicValueChallenge.create(req)
+
+        assert challenge.name == 'second_name'
+        assert challenge.description == "new_description"
+        assert challenge.value == 200
+        assert challenge.initial == 200
+        assert challenge.decay == 40
+        assert challenge.minimum == 5
+        assert challenge.state == "visible"
+
+        challenge_data = {
+            "requirements": [1]
+        }
+
+        req = FakeRequest(form=challenge_data)
+        challenge = DynamicValueChallenge.update(challenge, req)
+
+        assert challenge.requirements == [1]
+
+    destroy_ctfd(app)
+
+
 def test_can_delete_dynamic_challenge():
     app = create_ctfd(enable_plugins=True)
     with app.app_context():