smartlist.rs: The libc::c_char type is not the same as i8.

The code had been using c_char and i8 interchangeably, but it turns
out that c_char is only i8 on platforms where "char" is signed. On
other platforms, c_char is u8.

Fixes bug 24794; bug not on any released Tor.

src/rust/smartlist/smartlist.rs

@@ -33,7 +33,7 @@ impl Smartlist<String> for Stringlist {
 
         // unsafe, as we need to extract the smartlist list into a vector of
         // pointers, and then transform each element into a Rust string.
-        let elems: &[*const i8] =
+        let elems: &[*const c_char] =
             unsafe { slice::from_raw_parts(self.list, self.num_used as usize) };
 
         for elem in elems.iter() {