Fix potential buffer overflow in insert_many

Fixes servo#252.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "smallvec"
-version = "1.6.0"
+version = "1.6.1"
 edition = "2018"
 authors = ["The Servo Project Developers"]
 license = "MIT/Apache-2.0"

src/lib.rs

@@ -1041,7 +1041,10 @@ impl<A: Array> SmallVec<A> {
                 let mut cur = ptr.add(num_added);
                 if num_added >= lower_size_bound {
                     // Iterator provided more elements than the hint.  Move trailing items again.
+                    self.set_len(guard.len);
                     self.reserve(1);
+                    self.set_len(0);
+
                     let start = self.as_mut_ptr();
                     ptr = start.add(index);
                     cur = ptr.add(num_added);

src/tests.rs

@@ -905,3 +905,16 @@ fn empty_macro() {
 fn zero_size_items() {
     SmallVec::<[(); 0]>::new().push(());
 }
+
+#[test]
+fn test_insert_many_overflow() {
+    let mut v: SmallVec<[u8; 1]> = SmallVec::new();
+    v.push(123);
+
+    // Prepare an iterator with small lower bound
+    let iter = (0u8..5).filter(|n| n % 2 == 0);
+    assert_eq!(iter.size_hint().0, 0);
+
+    v.insert_many(0, iter);
+    assert_eq!(&*v, &[0, 2, 4, 123]);
+}