Questions without coreference should not carry over slots

[gcampax]

From a tester's log:

U: what is the biggest hit by Taylor swift
UT: $dialogue @org.thingpedia.dialogue.transaction.execute;
UT: sort(popularity desc of @com.spotify.song() filter contains(artists, null^^com.spotify:artist("taylor swift")))[1];
A: The most popular track is willow. It is a track in the album evermore by Taylor Swift.
AT: $dialogue @org.thingpedia.dialogue.transaction.sys_recommend_one;
U: who sings shake it off
UT: $dialogue @org.thingpedia.dialogue.transaction.execute;
UT: [artists] of sort(popularity desc of @com.spotify.song() filter contains(artists, "spotify:artist:06HL4z0CvFAxyc27GXpf02"^^com.spotify:artist("Taylor Swift")) && id == "spotify:track:0lx2cLdOt3piJbcaXIV74f"^^com.spotify:song("willow"))[1];

The second parse is incorrect, it should be:

[artists] of @com.spotify.song() filter id =~ "shake it off";

The second parse arises because we carry over the slots, due to bad training data.