Place colon for correct zdict using the last 32k bytes

Author: rhpvorderman

src/isal/igzip_threaded.py

@@ -177,7 +177,7 @@ def write(self, b) -> int:
             raise IOError("Can not write closed file")
         index = self.index
         data = bytes(b)
-        zdict = memoryview(self.previous_block)[:-DEFLATE_WINDOW_SIZE]
+        zdict = memoryview(self.previous_block)[-DEFLATE_WINDOW_SIZE:]
         self.previous_block = data
         self.index += 1
         worker_index = index % self.threads