winmm_midi: unprepare the earlier buffers when one fails (#19766)

When midiOutPrepareHeader failed on buffer i, the unwind loop
"while (--i <= 0)" ran on an unsigned i: a failure on buffer 2 left
buffers 0 and 1 prepared, and a failure on buffer 1 unprepared buffer
0 and stopped only because i wrapped. Count down with i-- > 0.

Tests: under wine, with a harness that includes winmm_midi.c and fakes
the two header calls. Before: a failure on the last buffer gives
prepared 2, unprepared 0. After: a failure on buffer 0, 1 or 2
unprepares exactly what was prepared. Cross-compiled with
x86_64-w64-mingw32-gcc -Wall -Wsign-compare, no warnings.

Co-authored-by: Claude Opus 5.5 <[email protected]>

midi/drivers/winmm_midi.c

@@ -287,7 +287,7 @@ static bool winmm_midi_init_output_buffers(HMIDISTRM dev,
       {
          RARCH_ERR("[MIDI] midiOutPrepareHeader failed with error %d.\n", mmr);
 
-         while (--i <= 0)
+         while (i-- > 0)
             midiOutUnprepareHeader((HMIDIOUT)dev, &bufs[i].header, sizeof(MIDIHDR));
 
          return false;