lnrpc/invoicesrpc: clamp down on max invoice size
lnrpc/invoicesrpc: clamp down on max invoice size

In this commit, we move to clamp down somewhat on the max invoice size
after the limit was removed as part of the mpp changes. In #4210, it was
reported that a value of -1, would underflow and end up as 18 million
BTC, which would trip checks w.r.t the max expressible value in mSAT.
In this commit, we clamp things down to 100k BTC, which should be more
than enough for anybody.

Fixes #4210.

aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd