url_preview: Discard url in oembed if server returns invalid json.

This fixes the scenario where we'd get errors in the
FetchLinksEmbedData queue processor if oembed got invalid json from the
URL.

zerver/lib/url_preview/oembed.py

@@ -1,12 +1,13 @@
 from typing import Optional, Dict, Any
 from pyoembed import oEmbed, PyOembedException
+import json
 
 def get_oembed_data(url: str,
                     maxwidth: Optional[int]=640,
                     maxheight: Optional[int]=480) -> Optional[Dict[str, Any]]:
     try:
         data = oEmbed(url, maxwidth=maxwidth, maxheight=maxheight)
-    except PyOembedException:
+    except (PyOembedException, json.decoder.JSONDecodeError):
         return None
 
     oembed_resource_type = data.get('type', '')

zerver/tests/test_link_embed.py

@@ -114,6 +114,16 @@ class OembedTestCase(ZulipTestCase):
         data = get_oembed_data(url)
         self.assertIsNone(data)
 
+    @mock.patch('pyoembed.requests.get')
+    def test_invalid_json_in_response(self, get: Any) -> None:
+        get.return_value = response = mock.Mock()
+        response.headers = {'content-type': 'application/json'}
+        response.ok = True
+        response.text = '{invalid json}'
+        url = 'http://instagram.com/p/BLtI2WdAymy'
+        data = get_oembed_data(url)
+        self.assertIsNone(data)
+
     def test_oembed_html(self) -> None:
         html = '<iframe src="//www.instagram.com/embed.js"></iframe>'
         stripped_html = strip_cdata(html)