DirectSound: use calloc for aout_stream_sys_t

Fix a crash occurring when CreateDSBufferPCM() fails in Start() and
afterwards Stop() uses uninitialized pointer values when releasing
objects.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

modules/audio_output/directsound.c

@@ -747,7 +747,7 @@ static HRESULT StreamStart( aout_stream_t *s,
                             audio_sample_format_t *restrict fmt,
                             const GUID *sid )
 {
-    aout_stream_sys_t *sys = malloc( sizeof( *sys ) );
+    aout_stream_sys_t *sys = calloc( 1, sizeof( *sys ) );
     if( unlikely(sys == NULL) )
         return E_OUTOFMEMORY;