Internal error panic when parsing "impl<x<>>::x for" malformed `syn::File`

[5225225]

Found by a fuzzer, it looks to me that fuzzer panics are valid as long as they're not stack overflows. (Considered adding a fuzzer with a length limit and a comment saying as much? Could also test for round-tripping to some extent). Tried to minimise it as much as I can, but i can't do any more than this.

Test case:

#[test]
fn parse() {
    let data = "impl<x<>>::x for";
    
    let _: Result<syn::File, _> = syn::parse_str(data);
}

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