Ensure reader buffer is flushed when extracting reader

After the decoder stream has yielded all of the uncompressed data, it is possible for the input stream to still not be fully consumed.  This means if we extract the inner stream at this point, it will not be pointing to the end of the compressed data.

From the [zstd documentation](https://facebook.github.io/zstd/zstd_manual.html#Chapter9) for `ZSTD_decompressStream`:

> But if `output.pos == output.size`, there might be some data left within internal buffers.
> In which case, call ZSTD_decompressStream() again to flush whatever remains in the buffer.

This is only necessary if the caller wants the stream back, so at that point we can force an additional call to `ZSTD_decompressStream` by reading to a zero-length buffer.

src/stream/read/mod.rs

@@ -93,7 +93,9 @@ impl<'a, R: BufRead> Decoder<'a, R> {
     ///
     /// Calling `finish()` is not *required* after reading a stream -
     /// just use it if you need to get the `Read` back.
-    pub fn finish(self) -> R {
+    pub fn finish(mut self) -> R {
+        // Ensure the input buffers have been flushed by reading to a zero-length buffer.
+        let _ = self.reader.read(&mut [0; 0]);
         self.reader.into_inner()
     }