suggestion-model: fix crash when typing in address bar

We have a refcounting error in ephy_suggestion_model_query_async(), where we pass our task into query_collections_done() without reffing it. Then the task is already destroyed when we go to form the history query, and it has destroyed our QueryData with it, and so we have a UI process crash when attempting to g_strsplit (data->query).

There's no need for each outstanding query to ref the task. Let's just use one ref and drop it when the final query completes.

Fixes #1522 (closed)