rustdoc segfault for invalid code block boundaries

[aidanhs]

$ cat seg.md 
% x

```rust

```a
$ rustdoc seg.md 
<stdin>:2:1: 2:2 error: unknown start of token: `
<stdin>:2 ```a
          ^
thread '<unnamed>' panicked at 'Box<Any>', /home/aidanhs/Desktop/rust/rust/src/libsyntax/diagnostic.rs:93
Segmentation fault (core dumped)

[alexcrichton]

It looks like this has been fixed in the meantime, yay!