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

[Huw Davies]

On 7 Sep 2016, at 11:24, Marvin <testbot at winehq.org> wrote:
> === 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

These are unrelated.

Interestingly, a quick google[1] suggested calling CoInitialiseEx() prior to calling
midiOutOpen(), and indeed doing so does fix these errors.

Huw.

[1] http://stackoverflow.com/questions/31752786/midioutopen-on-windows-10-using-microsoft-gs-wavetable-synth-fails