Improve the BaseStream.prototype.clone implementations

- The `dict` field is optional, hence avoid an Error if trying to clone a non-existent dictionary.

 - Use the `length` getter in the `Stream` class, to avoid duplication.

 - Fix the `DecodeStream` implementation, since it has a couple of bugs:
    - The `clone` method currently uses `start`/`end` fields, despite these only existing on `Stream` instances.
    - Given the previous point, we ended up creating the cloned `Stream` instance using the *entire* underlying `buffer`. This is problematic since the length of a `DecodeStream` cannot be accurately estimated before decoding, and the `buffer`-length is simply a multiple of two.
       Unless the size of the decoded-data just happens to also be a multiple of two, this causes the cloned `Stream` instance to be "padded" with zeros at the end.

src/core/base_stream.js

@@ -135,6 +135,10 @@ class BaseStream {
     unreachable("Abstract method `makeSubStream` called");
   }
 
+  clone() {
+    unreachable("Abstract method `clone` called");
+  }
+
   /**
    * @returns {Array | null}
    */

src/core/decode_stream.js

@@ -178,21 +178,16 @@ class DecodeStream extends BaseStream {
     return new Stream(this.buffer, start, length, dict);
   }
 
-  getBaseStreams() {
-    return this.stream ? this.stream.getBaseStreams() : null;
-  }
-
   clone() {
     // Make sure it has been fully read.
     while (!this.eof) {
       this.readBlock();
     }
-    return new Stream(
-      this.buffer,
-      this.start,
-      this.end - this.start,
-      this.dict.clone()
-    );
+    return new Stream(this.buffer, 0, this.bufferLength, this.dict?.clone());
+  }
+
+  getBaseStreams() {
+    return this.stream ? this.stream.getBaseStreams() : null;
   }
 }
 

src/core/stream.js

@@ -88,8 +88,8 @@ class Stream extends BaseStream {
     return new Stream(
       this.bytes.buffer,
       this.start,
-      this.end - this.start,
-      this.dict.clone()
+      this.length,
+      this.dict?.clone()
     );
   }
 }