avcodec/qsvdec: Fix null dereferences in the qsv decoder

This patch fixes the h264_qsv decoder issues mentioned
in https://ffmpeg.zeranoe.com/forum/viewtopic.php?t=2962.

The patch may be tested by specifying h264_qsv as the decoder to ffplay
for an h264 encoded file.

	ffplay -vcodec h264_qsv foo.mts
Signed-off-by: Yuli Khodorkovskiy <ykhodo@gmail.com>
Push requested-by: Ivan Uskov
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/qsvdec.c

@@ -555,16 +555,18 @@ void ff_qsv_decode_reset(AVCodecContext *avctx, QSVContext *q)
     }
 
     /* Reset output surfaces */
-    av_fifo_reset(q->async_fifo);
+    if (q->async_fifo)
+        av_fifo_reset(q->async_fifo);
 
     /* Reset input packets fifo */
-    while (av_fifo_size(q->pkt_fifo)) {
+    while (q->pkt_fifo && av_fifo_size(q->pkt_fifo)) {
         av_fifo_generic_read(q->pkt_fifo, &pkt, sizeof(pkt), NULL);
         av_packet_unref(&pkt);
     }
 
     /* Reset input bitstream fifo */
-    av_fifo_reset(q->input_fifo);
+    if (q->input_fifo)
+        av_fifo_reset(q->input_fifo);
 }
 
 int ff_qsv_decode_close(QSVContext *q)