avcodec/mpegvideo_dec, h264_slice: Return proper error codes

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/h264_slice.c

@@ -483,7 +483,7 @@ static int h264_frame_start(H264Context *h)
 
     if (!ff_thread_can_start_frame(h->avctx)) {
         av_log(h->avctx, AV_LOG_ERROR, "Attempt to start a frame outside SETUP state\n");
-        return -1;
+        return AVERROR_BUG;
     }
 
     release_unused_pictures(h, 1);

libavcodec/mpegvideo_dec.c

@@ -328,7 +328,7 @@ int ff_mpv_frame_start(MpegEncContext *s, AVCodecContext *avctx)
 
     if (!ff_thread_can_start_frame(avctx)) {
         av_log(avctx, AV_LOG_ERROR, "Attempt to start a frame outside SETUP state\n");
-        return -1;
+        return AVERROR_BUG;
     }
 
     /* mark & release old frames */