dshow: cleanup reference

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

libavdevice/dshow.c

@@ -916,8 +916,8 @@ static int dshow_read_packet(AVFormatContext *s, AVPacket *pkt)
     while (!pktl) {
         WaitForSingleObject(ctx->mutex, INFINITE);
         pktl = ctx->pktl;
-        if (ctx->pktl) {
-            *pkt = ctx->pktl->pkt;
+        if (pktl) {
+            *pkt = pktl->pkt;
             ctx->pktl = ctx->pktl->next;
             av_free(pktl);
         }