cli: Fix priority fee calculation causing panic on localnet

[acheroncrypto]

Problem

https://github.com/coral-xyz/anchor/blob/880a1e98a31f48362d2402f0a3649309c84f3909/cli/src/lib.rs#L4735

Fails with:

thread 'main' panicked at cli/src/lib.rs:4735:15:
attempt to subtract with overflow
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

When there is no recent prioritization fees, e.g. on localnet.

Summary of changes

Fix priority fee calculation causing panic on localnet.

[vercel]

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