qsvdec: only access hwaccel_context is the pixel format is QSV

We do not strictly specify that hwaccel_context must be cleared if no
hwaccel is used.
Reported-By: wm4 <nfxjfg@googlemail.com>

libavcodec/qsvdec.c

@@ -106,7 +106,7 @@ static int qsv_decode_init(AVCodecContext *avctx, QSVContext *q)
             return AVERROR(ENOMEM);
     }
 
-    if (avctx->hwaccel_context) {
+    if (avctx->pix_fmt == AV_PIX_FMT_QSV && avctx->hwaccel_context) {
         AVQSVContext *user_ctx = avctx->hwaccel_context;
         session           = user_ctx->session;
         iopattern         = user_ctx->iopattern;