Check that the feature strings are well-formed

Assert that the feature strings are NUL terminated, so that they will
be well-formed as C strings.

This is a safety check to ease the maintaninace and update of the
feature lists.

src/librustc_driver/target_features.rs

@@ -54,6 +54,7 @@ pub fn add_configuration(cfg: &mut ast::CrateConfig, sess: &Session) {
 
     let tf = InternedString::new("target_feature");
     for feat in whitelist {
+        assert_eq!(feat.chars().last(), Some('\0'));
         if unsafe { LLVMRustHasFeature(target_machine, feat.as_ptr() as *const c_char) } {
             cfg.push(attr::mk_name_value_item_str(tf.clone(), intern(&feat[..feat.len()-1])))
         }