Fix MinHTLC/MaxHTLC calculation in dropchannelgraph

[guggero]

The min/max HTLC settings on the freshly inserted edge policies after dropping the graph seem to be incorrect.

[lncli] rpc error: code = Unknown desc = generated invalid channel update sig: max_htlc(20000000000 mSAT) for channel update greater than capacity(1000000000 mSAT)