fix: preserve byteLength when cloning a DataView

copyBuffer routed DataView through the TypedArray path, passing cur.length
(undefined on a DataView) as the constructor's third argument. Since that
argument is a byte length for DataView, a partial view was extended to the
end of the buffer. Clone DataView explicitly using byteLength.

index.js

@@ -6,6 +6,12 @@ function copyBuffer (cur) {
     return Buffer.from(cur)
   }
 
+  // A DataView has no `length`; its third constructor argument is a byte
+  // length, so use `byteLength`. TypedArrays take an element count via `length`.
+  if (cur instanceof DataView) {
+    return new DataView(cur.buffer.slice(), cur.byteOffset, cur.byteLength)
+  }
+
   return new cur.constructor(cur.buffer.slice(), cur.byteOffset, cur.length)
 }