fix underflow in vec swap_remove

fixes #16200

src/libcollections/vec.rs

@@ -964,7 +964,7 @@ impl<T> Vec<T> {
     #[inline]
     pub fn swap_remove(&mut self, index: uint) -> Option<T> {
         let length = self.len();
-        if index < length - 1 {
+        if length > 0 && index < length - 1 {
             self.as_mut_slice().swap(index, length - 1);
         } else if index >= length {
             return None
@@ -2003,6 +2003,12 @@ mod tests {
         let _ = vec[3];
     }
 
+    #[test]
+    fn test_swap_remove_empty() {
+        let mut vec: Vec<uint> = vec!();
+        assert_eq!(vec.swap_remove(0), None);
+    }
+
     #[bench]
     fn bench_new(b: &mut Bencher) {
         b.iter(|| {