winmm/tests: Pass the full size of MIDIHDR to midiOutPrepareHeader().

[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
https://testbot.winehq.org/JobDetails.pl?Key=25634

Your paranoid android.


=== w1064 (32 bit midi) ===
midi.c:290: Test failed: midiOutOpen(dev=0) rc=MMSYSERR_ERROR
midi.c:489: Test failed: midiStreamOpen(dev=-2) rc=MMSYSERR_ERROR
midi.c:290: Test failed: midiOutOpen(dev=-1) rc=MMSYSERR_ERROR
midi.c:489: Test failed: midiStreamOpen(dev=-2) rc=MMSYSERR_ERROR

=== w1064 (64 bit midi) ===
midi.c:290: Test failed: midiOutOpen(dev=0) rc=MMSYSERR_ERROR
midi.c:489: Test failed: midiStreamOpen(dev=-2) rc=MMSYSERR_ERROR
midi.c:290: Test failed: midiOutOpen(dev=-1) rc=MMSYSERR_ERROR
midi.c:489: Test failed: midiStreamOpen(dev=-2) rc=MMSYSERR_ERROR