Don't abort on an empty input file

[Nishuuzz]

An empty input file aborts every tool that formats errors, in any build with assertions on:

$ : > empty.wat
$ wat2wasm empty.wat -o /dev/null
wat2wasm: src/lexer-source-line-finder.cc:30: wabt::LexerSourceLineFinder::LexerSourceLineFinder(...): Assertion `Succeeded(result)' failed.
Aborted

Same for wast2json and wat-desugar.

LexerSource::Seek refuses to seek to the end:

if (offset < size_) {

On a zero-length source that makes Seek(0) fail, and LexerSourceLineFinder's constructor asserts it succeeded. Seek has exactly that one caller, and ReadRange already clamps to size_, so accepting offset == size_ is consistent with the rest of the class — it's simply where an empty source starts, and reading from there yields nothing.

With that, the tools do what they already meant to. wat2wasm warns empty module and writes out an empty module, wast2json warns empty script, and wat-desugar reports no module in file — all of those paths already existed, the assert just fired before they could run.

The test is a unit test rather than one under test/parse, because the harness strips the ;;; directive lines and leaves a newline, so the input is never actually zero bytes. That's also why the existing test/parse/empty-file.txt doesn't catch this — its input is a comment, not nothing.

Found while fuzzing the text front end. For what it's worth, fuzzers/wat2wasm_fuzzer.cc parses but never formats errors, and the line finder is only constructed when formatting, so a zero-length input never reaches it there — which is presumably how this survived.

[sbc100]

Can this be a file-based test in test/parse/? Is it hard to make a completely empty file there?