mjpegb: Detect changing number of planes in interlaced video

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/mjpegdec.c

@@ -242,6 +242,13 @@ int ff_mjpeg_decode_sof(MJpegDecodeContext *s)
     if (nb_components <= 0 ||
         nb_components > MAX_COMPONENTS)
         return -1;
+    if (s->interlaced && (s->bottom_field == !s->interlace_polarity)) {
+        if (nb_components != s->nb_components) {
+            av_log(s->avctx, AV_LOG_ERROR,
+                   "nb_components changing in interlaced picture\n");
+            return AVERROR_INVALIDDATA;
+        }
+    }
     if (s->ls && !(s->bits <= 8 || nb_components == 1)) {
         avpriv_report_missing_feature(s->avctx,
                                       "JPEG-LS that is not <= 8 "