Correct handling of MPP overshooting

[TheBlueMatt]

See lightning/bolts#1031

[alecchendev]

I can take this on! I think I've found the spot in ChannelManager::process_pending_htlc_forwards where this is checked, so I'll try to open a PR soon to make this change and fix/write tests.

[valentinewallace]

Closed by #2062