dirmngr,w32: Fix ldap crl read on windows

Summary:

• dirmngr/ldap-wrapper-ce.c (outstream_cookie_s): Add buffer_read_pos.

(buffer_get_data): Use seperate read pos.

Using a single buffer pos for reading and writing caused the read
to return 0 as it read from the end of the buffer. Now we use
a seperate reader position.

Differential: D427

• Signed-off-by: Andre Heinecke <aheinecke@intevation.de>