Ensure fees are calculated without overflow

[dangeross]

Getting an overflow calculating the payment fees_sat. It's from a incoming chain swap that was underpaid. I have used saturating_sub to prevent the overflow.

thread 'tokio-runtime-worker' panicked at /Users/rosssavage/Source/Projects/breez-sdk-liquid/lib/core/src/model.rs:1560:23:
attempt to subtract with overflow
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This is the PaymentSwapData causing the issue:

[2025-01-17 14:56:33.613 DEBUG breez_sdk_liquid::model:1548] Swap ID pending: PaymentSwapData { swap_id: "83CFncYW8e8R", swap_type: Chain, created_at: 1733902652, expiration_blockheight: 874405, preimage: Some("408dd22e5bf613b4d055c22ad2a1a79ba112e03b95237a3dacd305388fcd83f1"), invoice: None, bolt12_offer: None, payment_hash: None, destination_pubkey: None, description: "Bitcoin transfer", payer_amount_sat: 20000, receiver_amount_sat: 47645, swapper_fees_sat: 0, refund_tx_id: Some("fa6699f193043de11e9542496799166b59020416383011f8923837665bde07c1"), refund_tx_amount_sat: None, claim_address: None, status: Failed }

[danielgranhao]

LGTM. Might be a good idea to log occurrences for debugging purposes, but we don't need to do it in this PR