avcodec/cuviddec: correctly set key_frame with interlaced content

Fixes #9283

This fixes setting of 'key_frame' flag in AVFrame when input h264 packets represents individual fields of interlaced video.
In this case, pairs of two consecutive fields represents a single decoded picture and have identical 'CurrPicIdx', however, only
the first field is entirely intra-coded and has the flag 'intra_pic_flag' set and the second field was resetting the flag before
it was even read in the function 'cuvid_output_frame'.

Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org>

libavcodec/cuviddec.c

@@ -336,7 +336,8 @@ static int CUDAAPI cuvid_handle_picture_decode(void *opaque, CUVIDPICPARAMS* pic
 
     av_log(avctx, AV_LOG_TRACE, "pfnDecodePicture\n");
 
-    ctx->key_frame[picparams->CurrPicIdx] = picparams->intra_pic_flag;
+    if(picparams->intra_pic_flag)
+        ctx->key_frame[picparams->CurrPicIdx] = picparams->intra_pic_flag;
 
     ctx->internal_error = CHECK_CU(ctx->cvdl->cuvidDecodePicture(ctx->cudecoder, picparams));
     if (ctx->internal_error < 0)
@@ -593,6 +594,8 @@ static int cuvid_output_frame(AVCodecContext *avctx, AVFrame *frame)
         }
 
         frame->key_frame = ctx->key_frame[parsed_frame.dispinfo.picture_index];
+        ctx->key_frame[parsed_frame.dispinfo.picture_index] = 0;
+
         frame->width = avctx->width;
         frame->height = avctx->height;
         if (avctx->pkt_timebase.num && avctx->pkt_timebase.den)