avcodec/qsvenc_h264: fix segfault when a53 SEI is not available

Signed-off-by: Nablet Developer <sdk@nablet.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/qsvenc_h264.c

@@ -53,7 +53,7 @@ static int qsv_h264_set_encode_ctrl(AVCodecContext *avctx,
         int res;
 
         res = ff_alloc_a53_sei(frame, sizeof(mfxPayload) + 2, (void**)&payload, &sei_size);
-        if (res < 0)
+        if (res < 0 || !payload)
             return res;
 
         sei_data = (mfxU8*)(payload + 1);