[wasm_split] Fix crash when manifest starts with a blank line

[sorensp-goog]

When the first line of the manifest file is blank, currFuncs is null, but will be dereferenced to check if the current module will be empty.

This CL fixes that and also cleans up the logic to clarify how the state machine works.

[stevenfontanella]

Can you give an example repro program + wasm-split invocation that demonstrates the crash, and ideally add a lit test as well? I see some wasm-split lit tests in test/lit/wasm-split. You can add a lit test by adding a RUN: line and then running

python3 scripts/update_lit_checks.py -f <test path>`

[sorensp-goog]

Can you give an example repro program + wasm-split invocation that demonstrates the crash, and ideally add a lit test as well?

There is no repro program as such, but any manifest that starts with a blank line will provoke a crash. I added a lit test that covers various aspect of this state machine, including this one.