dshow: protect access to curbufsize

Signed-off-by: rogerdpack <rogerpack2005@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavdevice/dshow.c

@@ -920,6 +920,7 @@ static int dshow_read_packet(AVFormatContext *s, AVPacket *pkt)
             *pkt = pktl->pkt;
             ctx->pktl = ctx->pktl->next;
             av_free(pktl);
+            ctx->curbufsize -= pkt->size;
         }
         ResetEvent(ctx->event);
         ReleaseMutex(ctx->mutex);
@@ -932,8 +933,6 @@ static int dshow_read_packet(AVFormatContext *s, AVPacket *pkt)
         }
     }
 
-    ctx->curbufsize -= pkt->size;
-
     return pkt->size;
 }