bibtex: don't panic on an empty @string macro (#1372)

* bibtex: don't panic on an empty @string macro

`scan_a_field_token_and_eat_white` inspects `str[0]` to collapse leading
whitespace at the start of a field value, where `str` is the expansion of a
macro token. That expansion can be the empty string — cryptobib defines over a
thousand venue name/address macros as "" (e.g. `@string{acispname = ""}`) — in
which case `str` is a zero-length slice and `str[0]` is out of bounds:

    thread 'main' panicked at crates/engine_bibtex/src/scan.rs:663:
    index out of bounds: the len is 0 but the index is 0

This makes tectonic unable to compile any paper that cites cryptobib. The
original WEB/C BibTeX reads `str_pool[tmp_ptr]` from a shared array here and so
reads one byte past harmlessly; the Rust port returns an exact-length slice,
turning the same access into a panic. The copy loop a few lines below already
guards with `!str.is_empty()` — the leading-whitespace check simply didn't.

Guard it the same way. An empty macro then contributes nothing to the field,
matching reference bibtex/bibtex8.

Adds a regression test (tests/bibtex/cites/empty_macro) whose .bbl is
byte-identical to bibtex8's output.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Remove comment

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: osdnk

crates/engine_bibtex/src/scan.rs

@@ -660,6 +660,7 @@ fn scan_a_field_token_and_eat_white(
                     let mut str = globals.pool.get_str(strnum);
 
                     if globals.buffers.offset(BufTy::Ex, 1) == 0
+                        && !str.is_empty()
                         && LexClass::of(str[0]) == LexClass::Whitespace
                     {
                         if globals.buffers.offset(BufTy::Ex, 1) >= globals.buffers.len() {

tests/bibtex.rs

@@ -96,6 +96,11 @@ fn test_single_entry() {
     TestCase::new(&["cites", "single_entry"]).go()
 }
 
+#[test]
+fn test_empty_macro() {
+    TestCase::new(&["cites", "empty_macro"]).go()
+}
+
 #[test]
 fn test_odd_strings() {
     TestCase::new(&["cites", "odd_strings"])

tests/bibtex/cites/empty_macro.aux

@@ -0,0 +1,5 @@
+\relax
+\citation{Empty06}
+\bibdata{empty_macro}
+\bibcite{Empty06}{1}
+\bibstyle{../plain}

tests/bibtex/cites/empty_macro.bbl

@@ -0,0 +1,8 @@
+\begin{thebibliography}{1}
+
+\bibitem{Empty06}
+Nobody Jr.
+\newblock My article.
+\newblock Available online, 2006.
+
+\end{thebibliography}

tests/bibtex/cites/empty_macro.bib

@@ -0,0 +1,7 @@
+@string{empty = ""}
+
+@misc{ Empty06,
+       author = "Nobody Jr",
+       title = "My Article",
+       howpublished = empty # "Available online",
+       year = "2006" }

tests/bibtex/cites/empty_macro.blg

@@ -0,0 +1,5 @@
+This is BibTeX, Version 0.99d
+Capacity: max_strings=35307, hash_size=35307, hash_prime=30011
+The top-level auxiliary file: empty_macro.aux
+The style file: ../plain.bst
+Database file #1: empty_macro.bib