[Bug] move coverage tool panics

[igor-aptos]

🐛 Bug

aptos move coverage source fails with:
thread 'main' panicked at third_party/move/tools/move-coverage/src/source_coverage.rs:327:71:
attempt to subtract with overflow

To reproduce

check out igor/repro_coverage_failure branch

go to:
cd aptos-move/framework/aptos-stdlib
run:

cargo run -p aptos --release -- move test --move-2 --filter big_ordered_map --coverage
cargo run -p aptos --release -- move coverage source --module big_ordered_map

And it will fail with:
thread 'main' panicked at third_party/move/tools/move-coverage/src/source_coverage.rs:327:71:
attempt to subtract with overflow

Expected Behavior

A clear and concise description of what you expected to happen.

System information

Please complete the following information:

• Aptos Core Version
• Rust Version
• Computer OS

Additional context

Add any other context about the problem here.

[vineethk]

Fixed in #15857.