windowscodecs/tests: COM cleanup for converter.c.

[Marvin]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
http://testbot.winehq.org/JobDetails.pl?Key=8249

Your paranoid android.


=== WXPPROSP3 (32 bit converter) ===
No test summary line found

=== WVISTAADM (32 bit converter) ===
converter: unhandled exception c0000005 at 7749794D

=== W2K8SE (32 bit converter) ===
converter: unhandled exception c0000005 at 778A8C6B