Fix null pointer exception when multiple paths have the same signature

Author: quen2404

src/main/java/com/qdesrame/openapi/diff/compare/PathsDiff.java

@@ -38,6 +38,9 @@ public Optional<ChangedPaths> diff(final Map<String, PathItem> left, final Map<S
             String template = normalizePath(url);
             Optional<String> result = right.keySet().stream().filter(s -> normalizePath(s).equals(template)).findFirst();
             if (result.isPresent()) {
+                if (!changedPaths.getIncreased().containsKey(result.get())) {
+                    throw new IllegalArgumentException("Two path items have the same signature: " + template);
+                }
                 PathItem rightPath = changedPaths.getIncreased().remove(result.get());
                 Map<String, String> params = new HashMap<>();
                 if (!url.equals(result.get())) {

src/test/java/com/qdesrame/openapi/test/PathDiffTest.java

@@ -8,10 +8,16 @@ public class PathDiffTest {
 
     private final String OPENAPI_PATH1 = "path_1.yaml";
     private final String OPENAPI_PATH2 = "path_2.yaml";
+    private final String OPENAPI_PATH3 = "path_3.yaml";
 
     @Test
     public void testEqual() {
         assertOpenApiAreEquals(OPENAPI_PATH1, OPENAPI_PATH2);
     }
 
+    @Test(expected = IllegalArgumentException.class)
+    public void testMultiplePathWithSameSignature() {
+        assertOpenApiAreEquals(OPENAPI_PATH3, OPENAPI_PATH3);
+    }
+
 }

src/test/resources/path_3.yaml

@@ -0,0 +1,52 @@
+openapi: 3.0.0
+servers:
+  - url: 'http://petstore.swagger.io/v2'
+info:
+  description: >-
+    This is a sample server Petstore server.  You can find out more about
+    Swagger at [http://swagger.io](http://swagger.io) or on [irc.freenode.net,
+    #swagger](http://swagger.io/irc/).  For this sample, you can use the api key
+    `special-key` to test the authorization filters.
+  version: 1.0.0
+  title: Swagger Petstore
+  termsOfService: 'http://swagger.io/terms/'
+  contact:
+    email: apiteam@swagger.io
+  license:
+    name: Apache 2.0
+    url: 'http://www.apache.org/licenses/LICENSE-2.0.html'
+paths:
+  /pet/{petId}:
+    get:
+      tags:
+        - pet
+      summary: gets a pet by id
+      description: ''
+      operationId: updatePetWithForm
+      parameters:
+        - name: petId
+          in: path
+          description: ID of pet that needs to be updated
+          required: true
+          schema:
+            type: integer
+      responses:
+        '405':
+          description: Invalid input
+  /pet/{petId2}:
+    get:
+      tags:
+        - pet
+      summary: gets a pet by id
+      description: ''
+      operationId: updatePetWithForm
+      parameters:
+        - name: petId2
+          in: path
+          description: ID of pet that needs to be updated
+          required: true
+          schema:
+            type: integer
+      responses:
+        '405':
+          description: Invalid input