fix UB in VecDeque::shrink_to

Author: kadiwa4

Diff for: library/alloc/src/collections/vec_deque/mod.rs

@@ -981,7 +981,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
                 unsafe {
                     self.copy_nonoverlapping(target_cap, 0, len);
                 }
-            } else if self.head >= target_cap {
+            } else if !self.is_contiguous() {
                 //  H := head
                 //  L := last element
                 //            L                   H

Diff for: library/alloc/src/collections/vec_deque/tests.rs

@@ -1130,3 +1130,15 @@ fn issue_80303() {
     assert_eq!(vda, vdb);
     assert_eq!(hash_code(vda), hash_code(vdb));
 }
+
+#[test]
+fn issue_108453_shrink_to() {
+    let mut deque = VecDeque::with_capacity(10);
+
+    deque.push_back(1u8);
+    deque.push_front(10);
+    deque.push_front(9);
+    deque.shrink_to(9);
+
+    assert_eq!(deque, [9, 10, 1]);
+}