[instagram] default posts like_count to zero

I don't know when/why this happens and for private account reasons can't
add an example here, but i had this error out and it feels like a safe
fix to default to `0` here.

gallery_dl/extractor/instagram.py

@@ -165,7 +165,7 @@ class InstagramExtractor(Extractor):
             data = {
                 "post_id" : post["pk"],
                 "post_shortcode": post["code"],
-                "likes": post.get("like_count"),
+                "likes": post.get("like_count", 0),
                 "pinned": post.get("timeline_pinned_user_ids", ()),
                 "date": text.parse_timestamp(post.get("taken_at")),
             }