returning ScanError instead of running unreachable!()

fixes #23

saphyr/src/scanner.rs

@@ -949,7 +949,7 @@ impl<T: Iterator<Item=char>> Scanner<T> {
             self.roll_indent(mark.col, None, TokenType::BlockSequenceStart, mark);
         } else {
             // - * only allowed in block
-            unreachable!();
+            return Err(ScanError::new(self.mark, r#""-" is only valid inside a block"#))
         }
         try!(self.remove_simple_key());
         self.allow_simple_key();

saphyr/src/yaml.rs

@@ -469,4 +469,11 @@ a1: &DEFAULT
         assert!(doc[25][0].as_bool().unwrap());
         assert!(!doc[25][1].as_bool().unwrap());
     }
+
+    #[test]
+    fn test_bad_hypen() {
+        // See: https://github.com/chyh1990/yaml-rust/issues/23
+        let s = "{-";
+        assert!(YamlLoader::load_from_str(&s).is_err());
+    }
 }