loaders: ZIP: Seek to the end of the Zlib IO.

We need to Seek to the end of the Zlib I/O because:

* Files that are not images are not read at all

* There may be bytes that are not consumed by the loader
  (known to happen)

and we have to get to the next ZIP header.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>