Bump zlib version to 1.3

Bump zlib version to 1.3

We have been pulling in all actual changes over the past months, so this
is only a version number bump. The relevant entries of the ChangeLog are:

  * Building using K&R (pre-ANSI) function definitions is no longer supported.
  * Fixed a bug in deflateBound() for level 0 and memLevel 9.
  * Fixed a bug when gzungetc() is used immediately after gzopen().
  * Fixed a bug when using gzflush() with a very small buffer.
  * Fixed a crash when gzsetparams() is attempted for a transparent write.

ok deraadt miod millert