test2.py: fail if a downloaded image can't be decoded

modules/python/test/test2.py

@@ -13,7 +13,9 @@ class NewOpenCVTests(unittest.TestCase):
     def get_sample(self, filename, iscolor = cv.CV_LOAD_IMAGE_COLOR):
         if not filename in self.image_cache:
             filedata = urllib.urlopen("https://raw.github.com/Itseez/opencv/2.4/" + filename).read()
-            self.image_cache[filename] = cv2.imdecode(np.fromstring(filedata, dtype=np.uint8), iscolor)
+            image = cv2.imdecode(np.fromstring(filedata, dtype=np.uint8), iscolor)
+            self.assertFalse(image is None)
+            self.image_cache[filename] = image
         return self.image_cache[filename]
 
     def setUp(self):