Fix subtraction overflow when calculating index for `MultiValue::get()`.

Return `None` instead of panic in debug mode.
Fixes #232

src/value.rs

@@ -197,7 +197,6 @@ pub struct MultiValue<'lua>(Vec<Value<'lua>>);
 
 impl<'lua> MultiValue<'lua> {
     /// Creates an empty `MultiValue` containing no values.
-    #[inline]
     pub const fn new() -> MultiValue<'lua> {
         MultiValue(Vec::new())
     }
@@ -276,7 +275,10 @@ impl<'lua> MultiValue<'lua> {
 
     #[inline]
     pub fn get(&self, index: usize) -> Option<&Value<'lua>> {
-        self.0.get(self.0.len() - index - 1)
+        if index < self.0.len() {
+            return self.0.get(self.0.len() - index - 1);
+        }
+        None
     }
 
     #[inline]

tests/value.rs

@@ -1,6 +1,6 @@
 use std::ptr;
 
-use mlua::{Lua, Result, Value};
+use mlua::{Lua, MultiValue, Result, Value};
 
 #[test]
 fn test_value_eq() -> Result<()> {
@@ -63,3 +63,21 @@ fn test_value_eq() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn test_multi_value() {
+    let mut multi_value = MultiValue::new();
+    assert_eq!(multi_value.len(), 0);
+    assert_eq!(multi_value.get(0), None);
+
+    multi_value.push_front(Value::Number(2.));
+    multi_value.push_front(Value::Number(1.));
+    assert_eq!(multi_value.get(0), Some(&Value::Number(1.)));
+    assert_eq!(multi_value.get(1), Some(&Value::Number(2.)));
+
+    assert_eq!(multi_value.pop_front(), Some(Value::Number(1.)));
+    assert_eq!(multi_value[0], Value::Number(2.));
+
+    multi_value.clear();
+    assert!(multi_value.is_empty());
+}