patch 8.1.1605: Vim may delay processing messages on a json channel

Problem:    Vim may delay processing messages on a json channel. (Pontus
            Leitzler)
Solution:   Try parsing json when checking if there is readahead.
diff --git a/src/channel.c b/src/channel.c
index 7671e19..eb5af0a 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -2851,9 +2851,13 @@
     if (ch_mode == MODE_JSON || ch_mode == MODE_JS)
     {
 	jsonq_T   *head = &channel->ch_part[part].ch_json_head;
-	jsonq_T   *item = head->jq_next;
 
-	return item != NULL;
+	if (head->jq_next == NULL)
+	    // Parse json from readahead, there might be a complete message to
+	    // process.
+	    channel_parse_json(channel, part);
+
+	return head->jq_next != NULL;
     }
     return channel_peek(channel, part) != NULL;
 }