cli: Fix priority fee calculation causing panic on localnet

[acheroncrypto]

Problem

anchor/cli/src/lib.rs

Line 4735 in 880a1e9

(fees[median_index - 1].prioritization_fee + fees[median_index].prioritization_fee) / 2

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]

@acheroncrypto is attempting to deploy a commit to the coral-xyz Team on Vercel.

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