It's possible to cast `0 as extern fn()` in safe code

[bluss]

Rust allows casting 0 to a function pointer type like extern "Rust" fn(). This is unsafe.

rusti: let f = 0 as extern "Rust" fn() -> int; f()
-rusti:#rust- application terminated abnormally with signal 11 (Segmentation fault)

[bluss]

dup of #8730

[huonw]

Disagree that this is a dup; the other should be made specific to the type inference, since this is a glaring hole and not at all specific to extern "C" fn (like that issue).

cc @nikomatsakis.

[nikomatsakis]

Sounds like a bug all right.