std::at_vec: Fix segfault on overflow when resizing ~[@T]

Easy to reproduce:

    let mut v = ~[@1];
    v.resize(-1);  // success a.k.a silent failure
    v.push(@2); // segfault

src/libstd/at_vec.rs

@@ -230,13 +230,16 @@ pub unsafe fn reserve<T>(v: &mut @[T], n: uint) {
     // Implementation detail. Shouldn't be public
     #[allow(missing_doc)]
     pub fn reserve_raw(ty: *TyDesc, ptr: *mut *mut Box<Vec<()>>, n: uint) {
-
+        // check for `uint` overflow
         unsafe {
-            let size_in_bytes = n * (*ty).size;
-            if size_in_bytes > (**ptr).data.alloc {
-                let total_size = size_in_bytes + sys::size_of::<Vec<()>>();
+            if n > (**ptr).data.alloc / (*ty).size {
+                let alloc = n * (*ty).size;
+                let total_size = alloc + sys::size_of::<Vec<()>>();
+                if alloc / (*ty).size != n || total_size < alloc {
+                    fail!("vector size is too large: %u", n);
+                }
                 (*ptr) = local_realloc(*ptr as *(), total_size) as *mut Box<Vec<()>>;
-                (**ptr).data.alloc = size_in_bytes;
+                (**ptr).data.alloc = alloc;
             }
         }
 

src/libstd/vec.rs

@@ -3659,6 +3659,14 @@ fn test_overflow_does_not_cause_segfault() {
         v.push(2);
     }
 
+    #[test]
+    #[should_fail]
+    fn test_overflow_does_not_cause_segfault_managed() {
+        let mut v = ~[@1];
+        v.reserve(-1);
+        v.push(@2);
+    }
+
     #[test]
     fn test_mut_split() {
         let mut values = [1u8,2,3,4,5];