chanmon_consistency found force-close

The test `39ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff343235302a2a2a2a2a2a34343434343434343434343434343434343434fffe` on top of the changes in #753 manages to find a force-close where node 1 sends an HTLC which node 2 considers bogus as "Remote HTLC add would put them under remote reserve value". We (I mean Val) should fix this.