winealsa.drv: mixer.c: Initialize a variable in MIX_close [try 2]

[Zac Brown]

Initialize a variable in MIX_close.

Fixes valgrind bugs like:
Syscall param write(buf) points to uninitialised byte(s)
      ...
      by  MMDRV_Close (lolvldrv.c:352)
      by  mixerClose (winmm.c:401)
      by  mixer_test_deviceA (mixer.c:551)
      by  mixer_testsA (mixer.c:986)
      ...
    Address 0x7f21fa40 is on thread 1's stack
    Uninitialised value was created by a stack allocation
      at  MIX_Close (mixer.c:764)
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