Fix the assert condition to check the capacity.

Author: sunfishcode

src/buffer.rs

@@ -221,8 +221,8 @@ pub struct SpareCapacity<'a, T>(&'a mut Vec<T>);
 #[cfg(feature = "alloc")]
 #[cfg_attr(docsrs, doc(cfg(feature = "alloc")))]
 pub fn spare_capacity<'a, T>(v: &'a mut Vec<T>) -> SpareCapacity<'a, T> {
-    debug_assert!(
-        !v.is_empty(),
+    debug_assert_ne!(
+        v.capacity(), 0,
         "`extend` uses spare capacity, and never allocates new memory, so the `Vec` passed to it should have some spare capacity."
     );