ICE on invalid assert! invocation

[mitsuhiko]

This code currently causes an ICE:

assert!(1i32, 0i32);

Looks like this:

<std macros>:8:12: 21:21 error: mismatched types: expected `bool`, found `i32` (expected bool, found i32)
<std macros>:8         if !$cond {
<std macros>:9             fail!($($arg),+)
<std macros>:10         }
<std macros>:11     );
<std macros>:12 )
error: internal compiler error: unexpected failure
note: the compiler hit an unexpected failure path. this is a bug.
note: we would appreciate a bug report: http://doc.rust-lang.org/complement-bugreport.html
note: run with `RUST_BACKTRACE=1` for a backtrace
task 'rustc' failed at 'index out of bounds: the len is 12 but the index is 12', /build/rust-git/src/rust/src/libsyntax/lib.rs:1

[ghost]

Dupe of #15901

[alexcrichton]

Thanks @jakub-!