avcodec/mjpegdec: Fix context fields becoming inconsistent

Fixes out of array access
Fixes: asan_heap-oob_1ca4f85_2760_cov_144449187_miss_congeniality_pegasus_ljpg.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mjpegdec.c

@@ -1620,6 +1620,8 @@ static int mjpeg_decode_app(MJpegDecodeContext *s)
     }
 
     if (id == AV_RB32("LJIF")) {
+        int rgb = s->rgb;
+        int pegasus_rct = s->pegasus_rct;
         if (s->avctx->debug & FF_DEBUG_PICT_INFO)
             av_log(s->avctx, AV_LOG_INFO,
                    "Pegasus lossless jpeg header found\n");
@@ -1629,17 +1631,27 @@ static int mjpeg_decode_app(MJpegDecodeContext *s)
         skip_bits(&s->gb, 16); /* unknown always 0? */
         switch (i=get_bits(&s->gb, 8)) {
         case 1:
-            s->rgb         = 1;
-            s->pegasus_rct = 0;
+            rgb         = 1;
+            pegasus_rct = 0;
             break;
         case 2:
-            s->rgb         = 1;
-            s->pegasus_rct = 1;
+            rgb         = 1;
+            pegasus_rct = 1;
             break;
         default:
             av_log(s->avctx, AV_LOG_ERROR, "unknown colorspace %d\n", i);
         }
+
         len -= 9;
+        if (s->got_picture)
+            if (rgb != s->rgb || pegasus_rct != s->pegasus_rct) {
+                av_log(s->avctx, AV_LOG_WARNING, "Mismatching LJIF tag\n");
+                goto out;
+            }
+
+        s->rgb = rgb;
+        s->pegasus_rct = pegasus_rct;
+
         goto out;
     }
     if (id == AV_RL32("colr") && len > 0) {