Library terminates caller on error due to log.Fatalf

[its-luca]

In the attached snippet from bibtex.go:274 log.Fatalf is called to handle an undefined situation.
This gives the caller no chance to recover. I think this should be at least replaced with a panic instead.
Ideally, we could get some more robust error handling that i.e. returns invalid entries.
I tried to look into implementing this but failed to go generate bibtex.y.go using modernc.org/goyacc.
Is this the tool you used for generation?

// GetStringVar looks up a string by its key.
func (bib *BibTex) GetStringVar(key string) *BibVar {
	if bv, ok := bib.StringVar[key]; ok {
		return bv
	}
	if v, ok := bib.getDefaultVar(key); ok {
		return v
	}
	// This is undefined.
	log.Fatalf("%s: %s", ErrUnknownStringVar, key)
	return nil
}

Example triggering the log.Fatalf. (I know that this is invalid bibtex)

@misc{inteSDM,
  title={a},
  author={b},
  year={2019},
  month = May
}

Error message from go generate

panic: strings: negative Repeat count

goroutine 1 [running]:
strings.Repeat({0x102c51a20?, 0x16d2e798a?}, 0x1b?)
        /usr/local/go/src/strings/strings.go:533 +0x3a0
main.main1({0x16d2e79a0, 0x8})
        /Users/luca/go/pkg/mod/modernc.org/[email protected]/main.go:385 +0x294c
main.main()
        /Users/luca/go/pkg/mod/modernc.org/[email protected]/main.go:180 +0x110
lexer.go:1: running "goyacc": exit status 2