don't seek to end of destination in stream_copy_to_stream

- Doesn't break any stream related tests.
- php-src doesn't seem to do the seek:
  https://github.com/php/php-src/blob/8d4ee1dbedcd339f119f04058e72eebba272bdb5/main/streams/streams.c#L1496

See the test for an example of how this would break.

Fixes #2453.