Fix litcli sendpayment panic

[GeorgeTsagk]

Description

Sometimes the asset units may not be sufficient to carry out a payment (depending on the granularity and asset price). This may be lead to calculating "zero" units for a payment. This pr fixes a small piece of code that would blindly divide by numUnits without checking whether it's zero. If it's zero we need to error out as this means the payment cannot be carried out.

Solution

If we have numUnits equal to 0 we skip the division that would cause the panic, and instead return an error to the user which looks like this

Payment hash: 5cf59c0af46885e67054c8e0d5cf732952650e0965881db2e1e42d706528a78f
Description: 
Amount (in satoshis): 99
Fee limit (in satoshis): 99
Destination: 0327e3031fa45faa56be7e702de23e3d8c2305a5d69b410e2f44fa8bbca6dfa5ef
Confirm payment (yes/no): yes
[litcli] smallest payment with asset rate 1000000 is 100000 mSAT, cannot send 99000 mSAT

Related issues

Closes lightninglabs/taproot-assets#1230

[guggero]

tACK

While testing I noticed that the payment actually stays in-flight in the background forever. But that's unrelated to this change and needs to be fixed in lnd. Going to create a PR for it.

[Roasbeef]

LGTM 👑