Constrain the cursor and prevent segfaults

[rkfg]

This PR fixes two issues:

1. the cursor can move past the last channel or transaction; pressing Enter when it's not positioned on a valid item causes segmentation fault
2. the cursor can be positioned after the last channel/tx column; trying to sort while it's there also causes segfault

[edouardparis]

Thank you, It was well needed.
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[rkfg]

I created a tip page on lnpay.co: https://checkout.link/rkfg
tippin.me currently doesn't accept new members so I had to improvise. Thank you!