[vk] prevent exceptions for broken/invalid photos (#2774)

gallery_dl/extractor/vk.py

@@ -39,13 +39,18 @@ class VkExtractor(Extractor):
                 self.log.warning("no photo URL found (%s)", photo.get("id"))
                 continue
 
+            try:
+                photo["url"] = photo[size + "src"]
+            except KeyError:
+                self.log.warning("no photo URL found (%s)", photo.get("id"))
+                continue
+
             try:
                 _, photo["width"], photo["height"] = photo[size]
             except ValueError:
                 # photo without width/height entries (#2535)
                 photo["width"] = photo["height"] = 0
 
-            photo["url"] = photo[size + "src"]
             photo["id"] = photo["id"].rpartition("_")[2]
             photo.update(data)