failure in closure causing a pointer being freed was not allocated

[erickt]

Hi all,

I tracked down a malloc error with this code:

use std;
import io::{reader, reader_util};

fn main() {
    io::with_str_reader("") { |rdr|
        alt rdr.read_char() { '=' { } _ { fail } }
    }
}

Errors out with:

warning: no debug symbols in executable (-arch x86_64)
rust: upcall fail 'explicit failure', test2.rs:6
test2(64191,0x10061a000) malloc: *** error for object 0x100500040: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug
[1]    64191 abort      ./test2

[rwaldron]

cc @rwldrn

[brson]

Unsafe code in str::as_bytes was not coping with failure.