Panic / Crash in lex.rs line 296 when parsing malformed LaTeX input

### Description A fuzz test discovered a crash in the lexer when parsing specially crafted/malformed LaTeX input. The panic occurs in `lex.rs` at line 296. ### Reproduction 1. Use the attached minimal test case (or run the provided fuzz input) 2. Pass it to `pulldown-latex::parse` 3. The parser will panic at `lex.rs:296` ### Details - File: `lex.rs` - Line: 296 - Type: Panic / Crash - Impact: May cause denial-of-service (DoS) if untrusted input is parsed. ### Additional Information Found via libFuzzer-based fuzzing. The input is attached.