avdevice/dshow: Use av_freep() to avoid leaving stale pointers in memory

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavdevice/dshow.c

@@ -146,9 +146,9 @@ dshow_read_close(AVFormatContext *s)
         IBaseFilter_Release(ctx->device_filter[AudioDevice]);
 
     if (ctx->device_name[0])
-        av_free(ctx->device_name[0]);
+        av_freep(&ctx->device_name[0]);
     if (ctx->device_name[1])
-        av_free(ctx->device_name[1]);
+        av_freep(&ctx->device_name[1]);
 
     if(ctx->mutex)
         CloseHandle(ctx->mutex);