Fix MinHTLC/MaxHTLC calculation in dropchannelgraph

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)
```

Start by locating the dropchannelgraph entry point and tracing how min/max HTLC values are calculated for freshly inserted edge policies. Reproduce the reported invalid channel update signature and verify that the resulting max_htlc does not exceed the channel capacity while the min/max settings remain valid.

Written by the indexing model from the issue text.